Where do you get keys for Endless Arena??
You need to do monoliths to get the keys. Every 5th timeline you complete in the monolith you’ll get a key (timelines 5, 10, 15, etc) with a chance for a key in the in-between monoliths. The keys drop from the chest in the “echo of a world” (or wherever the portal takes you to when you complete the monolith). It’ll be a purple named item.