[Question]
  [
The idea of this challenge is to find all the words from a dictionary on a grid of letters. Your input should come from stdin or function arg and will come in the following format:
```
[your dictionary as a list of space sep words]
[a grid of letters]
Example:
The cool car
looc
thea
jopr
```
### Rules for finding a match
A match is valid if it is found diagonally, horizontally, or vertically on the board (case insensitive). The direction doesn't matter (e.g. `look` matches `kool`). If there are multiple matches of the same word, mark all of them.
### Output:
Your output will be the grid you were given - with the minor change that the letters will be space separated and the lines will be double spaced. Example:
```
Input:
looc
thea
jopr
Output:
l o o c
t h e a
j o p r
```
To represent a match you will put an arrow between letters that go together. The arrows (`/\-|X`) will point to each pair of letters that are grouped in a match. `X` is used if `/\` intersect.
### Examples:
```
Input:
The cool car
looc
thea
jopr
Output:
l-o-o-c
      |
t-h-e a
      |
j o p r
Input:
Atomic chess is cool
achess
btoikm
bloosi
nowmlp
hewiir
asdfec
Output:
a c-h-e-s-s
 \ \   /   
b t o i k m
   \ \ \   
b l o o s-i
     \ \   
n o w m l p
       \   
h e w i i r
         \ 
a s d f e c
Input:
This is very neat words var are fun rob bot robot
wotsdsearn
siiewfvery
chheruoawd
tetoennore
asbdrgrehe
aobyedycab
tweosttgwt
Output:
w o t s d s e a r n
     /             
s-i i e w f v-e-r-y
   /      |  \     
c h h e r u o a w d
 /     /  |    X   
t e t o e n n o r e
     /       X   \ 
a s b d r g r e h e
   /       /   \   
a o b y e d y c a b
 /       /       \ 
t w e o s t t g w t
```
  
[Answer]
# JavaScript (ES6), 303 ~~315~~
Mostly based on [this answer](https://codegolf.stackexchange.com/a/37974/21348)
**Edit 1**
* Following OP comment, ignoring casing and forcing all to lowercase.
* Fixed the arguments, now just a single string argument that get splitted
Note: using template strings, there are 3 newlines in the code that are significant and incuded in the byte count
```
l=>(l=l.toLowerCase().split`
`).shift(r=l[1].length*4).split` `.map(v=>[2,-2,r,-r,r+2,-r-2,r-2,2-r].map((d,k)=>o.map((t,p)=>[...v].some((c,i)=>z[i?(z[i=t+d/2]=z[i]>' '?'X':'-|\\/'[k>>1],t+=d):t=p]!=c,z=[...o])?0:o=z)),o=[...l.map(s=>' '.repeat(r/2-1)+`
${[...s].join` `}
`).join``.slice(r/2)])&&o.join``
```
**Explained** (outdated)
```
// Note a pattern : in golfed code (but not in the following explanation),
//                  simple operations that have to done before calling .map or .every
//                  are inserted as parameter 2,3,etc... to the same function
//                  as each parameter is evaluated before calling a function
// 
F=( l, // word list as a space separated string
    o, // character grid as a newline separated string - each row is the same length
    // default parameters used as local variables
    r = 4 * o.search`\n` // offset from a line to next nonblank line in resulting output
  )=>(
    w = ' ', // shortcut for blank
    // given the input string in o, build the result as a single dimension array filled with blanks
    // between characters and runs of blank chars betweem each row 
    // .map used as a shorter .forEach
    z = w, // init z to a single blank
    [...o].map( c=> // for each char of o execute the following
                c > w // check if c is a letter (>' ') or a newline
                 ? z+=w+c // if c is a letter, add ' '+c to z
                 : ( // if c is a newline add chars to array o
                     z = z.slice(2), // remove the first 2 char of z ('  ' or '\n ')
                     z = [...z], // convert to array
                     o.push(...z, // push one by one the chars of z
                            c, // then c (a newline)
                            ...z.fill(w), // the same number of char as z, but all blanks
                            z = c // a newline again, meanwhile reset z
                           )
                   )
            , o=[] // o is reset to an empty array just in the map call 
                   // reusing o just to avoid another global variable (no real need in fact)
    ), // end of .map call 
    l.split` `.map( // split l into words and exec the following for each word
      v => [2,-2,r,-r,r+2,-r-2,r-2,2-r].map( // for each scan direction
        (d,k) => // d is the move offset , k is the index 0 to 7
        o.map( // for each char in (including fill blanks and newlines, the scan will fail for them) 
          (t,p) => // t is the first character but NOT USED (var t reused), p is the start position
          (
            z=[...o], // make a copy of o in z
            t = p-d, // current position-d goes in t, it will be incremented before using
            [...v].every( // check the following conditions for every char of word v
            (c,i) => // c: current char of word, i: index used to differentiate the first check when i==0
              (
                // while scanning I already modify the result working copy in z
                i ? // if i != 0 
                  // fill the intermediate position with the right fiil character
                  // based on direction index in k, or X if the position is already full
                  z[i = t+d/2] = z[i] > w ? 'X' : '--||\\\\//'[k]
                : 0, // else do nothing
                // case insensitive comparison, if not a letter parseInt returns NaN that is != from any value
                // this is the condition retured to the .every function
                parseInt(c,36)==parseInt(o[t+=d],36) // meanwhile increment position in T
              )  
            ) ? // check the result of .every
            o = z // if true, update o from the working copy
            : 0 // if false do nothing
          ) // end of o.map operations
        ) // end of o.map function call
      ) // end of direction list map function call
    ) // end of l.split.map function call
    , o.join``
  )
 
```
**TEST**
```
console.log=(...x)=>O.textContent+=x.join` `+`\n`;
F=l=>
  (l=l.toLowerCase().split`\n`)
  .shift(r=l[1].length*4).split` `.map(v=>[2,-2,r,-r,r+2,-r-2,r-2,2-r].map((d,k)=>o.map((t,p)=>
    [...v].some((c,i)=>z[i?(z[i=t+d/2]=z[i]>' '?'X':'-|\\/'[k>>1],t+=d):t=p]!=c
    ,z=[...o])?0:o=z)
  ),o=[...l.map(s=>' '.repeat(r/2-1)+`\n${[...s].join` `}\n`).join``.slice(r/2)])
  &&o.join``
  
console.log(F('Atomic chess is cool\nachess\nbtoikm\nbloosi\nnowmlp\nhewiir\nasdfec'))
console.log(F("RANDOM VERTICAL HORIZONTAL WIKIPEDIA Tail WordSearch CODEGOLF UNICORN\nWVERTICALL\nROOAFFLSAB\nACRILIATOA\nNDODKONWDC\nDRKESOODDK\nOEEPZEGLIW\nMSIIHOAERA\nALRKRRIRER\nKODIDEDRCD\nHELWSLEUTH"))
```
```
<pre id=O></pre>
```
[Answer]
# Javascript (ES6), ~~908~~ ~~901~~ ~~609~~ ~~603~~ ~~556~~ 552 bytes
```
a=>(a=a.toLowerCase().replace(/\n/g,`
`).split`
`,b=a.slice(2).map(r=>r.length?r.replace(/./g,i=>i+` `).trim().split``:Array(a[2].length*2-1).fill(` `)),a[0].split` `.forEach(c=>{for(y=0;y<b.length;y+=2){for(x=0;x<b[y].length;x+=2){c[0]==b[y][x]&&(d=[-2,0,2]).map(l=>{d.map(k=>{e=`\\|/
-o-
/|\\`.split(`
`)[l/2+1][k/2+1];try{(f=(g,h,j)=>g>=c.length?1:(ch=c[g],((k||l)&&b[j+l][h+k]==ch&&f(g+1,h+k,j+l))?(b[j+l/2][h+k/2]=((b[j+l/2][h+k/2]=='\\'&&e=='/')||(b[j+l/2][h+k/2]=='/'&&e=='\\'))?'x':e):0))(1,x,y);}catch(e){}})})}}}),b.map(r=>r.join``).join`
`)
```
Ungolfed:
```
inp => (
    inp = inp.toLowerCase().replace(/\n/g, `
`).split `
`,
    puzzle = inp.slice(2).map(r => r.length ? r.replace(/./g, i => i + ` `).trim().split `` : Array(inp[2].length * 2 - 1).fill(` `)),
    inp[0].split ` `.forEach(word => {
        for (y = 0; y < puzzle.length; y += 2) {
            for (x = 0; x < puzzle[y].length; x += 2) {
                word[0] == puzzle[y][x] &&
                    (dirs = [-2, 0, 2]).map(ydir => {
                        dirs.map(xdir => {
                            symb = `\\|/
-o-
/|\\`.split(`
`)[ydir / 2 + 1][xdir / 2 + 1];
                            try {
                                (findWord = (chnum, xcoord, ycoord) =>
                                    chnum >= word.length ? 1 : (
                                        ch = word[chnum],
                                        ((xdir || ydir) && puzzle[ycoord + ydir][xcoord + xdir] == ch && findWord(chnum + 1, xcoord + xdir, ycoord + ydir)) ?
                                            (puzzle[ycoord + ydir / 2][xcoord + xdir / 2] = ((puzzle[ycoord + ydir / 2][xcoord + xdir / 2] == '\\' && symb == '/') || (puzzle[ycoord + ydir / 2][xcoord + xdir / 2] == '/' && symb == '\\')) ? 'x' : symb)
                                            : 0
                                    )
                                )(1, x, y);
                            } catch (e) {}
                        })
                    })
            }
        }
    }),
    puzzle.map(r => r.join ``).join `
`
)
```
**Test (should work with modern browsers that have some ES6 support):**
```
func=a=>(a=a.toLowerCase().replace(/\n/g,`
`).split`
`,b=a.slice(2).map(r=>r.length?r.replace(/./g,i=>i+` `).trim().split``:Array(a[2].length*2-1).fill(` `)),a[0].split` `.forEach(c=>{for(y=0;y<b.length;y+=2){for(x=0;x<b[y].length;x+=2){c[0]==b[y][x]&&(d=[-2,0,2]).map(l=>{d.map(k=>{e=`\\|/
-o-
/|\\`.split(`
`)[l/2+1][k/2+1];try{(f=(g,h,j)=>g>=c.length?1:(ch=c[g],((k||l)&&b[j+l][h+k]==ch&&f(g+1,h+k,j+l))?(b[j+l/2][h+k/2]=((b[j+l/2][h+k/2]=='\\'&&e=='/')||(b[j+l/2][h+k/2]=='/'&&e=='\\'))?'x':e):0))(1,x,y);}catch(e){}})})}}}),b.map(r=>r.join``).join`
`);
```
```
<textarea id="_input">Atomic chess is cool
achess
btoikm
bloosi
nowmlp
hewiir
asdfec</textarea>
<br />
<button onclick="document.getElementById('_output').innerHTML = func(document.getElementById('_input').value)">Run!</button>
<br />
<pre id="_output"></pre>
```
[Answer]
# Python 3, 1387
```
k=input().lower().split(" ")
a=[]
while 1:
 a.append(input())
 if not a[-1]:a.pop();break
b=sum([[list(' '.join(list(i))),[' ']*(len(i)*2-1)]for i in a],[])[:-1]
w,h=len(a[0]),len(a)
for l in range(h):
 r=a[l];c=list([1 if w in r else 2 if w in r[::-1] else 0, w] for w in k)
 for t,v in c:
  if not t:continue
  if t==2:v=v[::-1]
  i=r.index(v)
  for m in range(i,i+len(v)-1):b[l*2][m*2+1]="-"
for l in range(w):
 u=''.join(x[l]for x in a);_=list([1 if w in u else 2 if w in u[::-1]else 0,w]for w in k)
 for t,v in _:
  if not t:continue
  if t==2:v=v[::-1]
  i=u.index(v)
  for m in range(i,i+len(v)-1):b[m*2+1][l*2]="|"
def d(g,r=1,o=0):
 if g>=len(a[0]):o=g-w+1;g=w-1
 f=range(0,min(g+1,h-o));return[a[i+o][w-1-(g-i)if r else g-i]for i in f],[(i+o,w-1-(g-i)if r else g-i)for i in f]
for l in range(w+h-1):
 x,c=d(l);_=''.join(x);z=list([1 if w in _ else 2 if w in _[::-1]else 0,w]for w in k) 
 for t,v in z:
  if not t:continue
  if t==2:v=v[::-1]
  i=_.index(v)
  for m in range(i,i+len(v)-1):b[c[m][0]*2+1][c[m][1]*2+1]="\\"
for l in range(w+h-1):
 x,c=d(l,0);_=''.join(x);z=list([1 if w in _ else 2 if w in _[::-1]else 0,w]for w in k)
 for t,v in z:
  if not t:continue
  if t==2:v=v[::-1]
  i=_.index(v)
  for m in range(i,i+len(v)-1):y=c[m][0]*2+1;x=c[m][1]*2-1;j=b[y][x];b[y][x]="x"if j=="\\"else"/"
print('\n'.join(''.join(x) for x in b))
```
An "is" was missed in the second example
```
Atomic chess is cool
achess
btoikm
bloosi
nowmlp
hewiir
asdfec
a c-h-e-s-s
 \ \   /
b t o i k m
   \ \ \
b l o o s-i
     \ \
n o w m l p
       \
h e w i i r
         \
a s d f e c
```
Sorta ungolfed
```
words = input().lower().split(" ")
square = []
while 1:
 square.append(input())
 if not square[-1]:square.pop();break
solved = sum([[list(' '.join(list(i))),[' ']*(len(i)*2-1)]for i in square], [])[:-1]
w,h = len(square[0]), len(square)
for l in range(h):
 r = square[l]
 rm = list([1 if w in r else 2 if w in r[::-1] else 0, w] for w in words)
 for t,v in rm:
  if not t:continue
  v = v[::-1] if t==2 else v
  i = r.index(v)
  for m in range(i,i+len(v)-1):solved[l*2][m*2+1]="-"
for l in range(w):
 u = ''.join(x[l] for x in square)
 um = list([1 if w in u else 2 if w in u[::-1] else 0, w] for w in words)
 for t,v in um:
  if not t:continue
  v = v[::-1] if t==2 else v
  i = u.index(v)
  for m in range(i,i+len(v)-1):solved[m*2+1][l*2]="|"
def d(m,ind,r=1,o=0):
 if ind>=len(m[0]):o=ind-len(m[0])+1;ind=len(m[0])-1
 f=range(0,min(ind+1,len(m)-o));return[m[i+o][len(m[0])-1-(ind-i)if r else ind-i]for i in f],[(i+o,len(m[0])-1-(ind-i)if r else ind-i)for i in f]
for l in range(w+h-1):
 x,c = d(square,l)
 dl = ''.join(x)
 dlm = list([1 if w in dl else 2 if w in dl[::-1] else 0, w] for w in words)
 for t,v in dlm:
  if not t:continue
  v = v[::-1] if t==2 else v
  i = dl.index(v)
  for m in range(i,i+len(v)-1):solved[c[m][0]*2+1][c[m][1]*2+1]="\\"
for l in range(w+h-1):
 x,c = d(square,l,0)
 dr = ''.join(x)
 drm = list([1 if w in dr else 2 if w in dr[::-1] else 0, w] for w in words)
 for t,v in drm:
  if not t:continue
  v = v[::-1] if 
t==2 else v
  i = dr.index(v)
  for m in range(i,i+len(v)-1):y=c[m][0]*2+1;x=c[m][1]*2-1;j=solved[y][x];solved[y][x]="x"if j=="\\"else"/"
print('\n'.join(''.join(x) for x in solved))
```
[Answer]
# Mathematica, 478 bytes
```
f[j_,k_]:=({m,p}=ToCharacterCode@*StringSplit/@{j,ToLowerCase@k};
    t=Transpose;v=Length@m[[1]];i=MapIndexed;r=RotateLeft;
    y@d_:=t@i[r[PadLeft[#,2v d],#2 d]&,m];z[m_,d_,p_]:=i[r[#,#2d]&,t@m][[All,p]];
    c[m_,s_]:=(a=0~Table~{v};a[[Join@@(Range[#,#2-1]&@@@SequencePosition[#,s|Reverse@s])]]=1;a)&/@m;
    FromCharacterCode@Flatten@Riffle[#,10]&@
        Flatten[BitOr@@({{m,8m~c~#},{4t@c[t@m,#],2z[y@1~c~#,-1,-v;;-1]+z[c[y@-1,#],1,2;;v+1]}}&/@p)
            /.{0->32,1->47,2->92,3->88,4->124,8->45},{{3,1},{4,2}}])
```
Test case:
```
f["wotsdsearn\nsiiewfvery\nchheruoawd\ntetoennore\nasbdrgrehe\naobyedycab\ntweosttgwt",
    "This is very neat words var are fun rob bot robot"]
(*
w o t s d s e a r n 
     /              
s-i i e w f v-e-r-y 
   /      |  \      
c h h e r u o a w d 
 /     /  |    X    
t e t o e n n o r e 
     /       X   \  
a s b d r g r e h e 
   /       /   \    
a o b y e d y c a b 
 /       /       \  
t w e o s t t g w t 
*)
```
]