lol you cant die after you take the quest so you should wait until your strong enough to beat the cave.. my char that capped 2 or 3 times a long time ago when i took it was a bit hard to beat it.. mainly cause i tried to kill everything XD
You do fail it if you die. Which I didn't know until I had to do it 3 fricking times x_x but finally I figured out that you can't die outside of it either... sigh... it sucked.
some tar pits outside the legendary cave makes you fail the quest , I dont know if this is fixed but fi it isnt can you fix it . Its not nice to fall in a tar pit by mistake and fail legendary