You’ll find the chest in the Hall of Souls, under the summoning platform in front of the Court of Oryx. When the events aren’t happening there, the floor is charged with something that does damage to you over time. Your best bet is to loot the chest when you’re there with a bunch of people, using the statues and starting the event.
How to get Key of Gnashing Teeth
The key can be obtained like most other keys – by using the Wormfeeder rune and killing 50 enemies without dying. To get said rune, you’ll first have to have a Wormsinger rune. When you’re done, you’ll be rewarded with a random key. If it’s not the one you’re after, repeat the whole thing. The other way to get the key is by waiting for it to randomly drop during Patrol.