Help me figure out this game

Tiller

So I'm trying to remember this MMO dungeon or event I played in but can't remember the  game it was in. I think it was open world, but it could have been instanced. Not sure if it was GW2 and some hero challenge, Fractal or event in Heart of thorns, EQ2, SWTOR, BDO, Aion or Rift.

Some details I remember....It was this creepy smallish oval shaped den/cave with a pretty low roof, stalagmite and stalactite columns, and unbelievably dark with some roots hanging down. As you came around the corner you couldn't really make out much of anything except this smallish white patch of misty fog where some sort of a giant lizard spawned in and awaited a fight.

I remember having to go in a few times, and being relieved at seeing a few other players in there tackling the challenge. What ever it was it was pretty much instant death as people around were being killed rapidly, which forced you to start over. I remember having to stay ranged to fight it, but because of the AO and the small size of the dungeon it was impossible to avoid damage. I completed it, and happily left never  wanting to go back to whatever it was. Anyone have any ideas what it might be? I might be able to offer more details if needed.