Got stuck in Chaos Sanctuary, now what do I do?
The short answer is you were too low level / not geared up enough for the Chaos Sanctuary.
As pixel noted in the comment, you can leave your game and re-enter to have your most recent (or highest valued) corpse returned to you in town. It's best not to wait until you have a lot of corpses, if you need to do it, just do it.
Nextly, you need to go back and run act 3/4 again and find some better gear before attempting the Sanctuary. It's quite common in the Diablo franchise that you don't find the gear you need until after it's useless to you but it's a cross we have to bear. A few levels will help.
Beyond that you'd have to expand your question with your current skill tree, gear and stats for us to give you much more specific advice.
To reduce the chances of this in the future, before you break either of the two seals on the lower-right, have a town portal open right next to the seal.
As soon as you break the seal, if you see venom lords, go through the portal. Then come back by the waypoint and run all the way to the Chaos Sanctuary again. (This will be quick because you've already killed all the monsters on the way.)
You'll still have to fight the venom lords, but you'll at least have a lot more space to do it in.
Two comments:
- If you seriously get 1-hit death by the venom lords then your equipment is too low, your life is too low, your level is too low, and your fire resist is too low. Even if you manage to kill them eventually by chipping away at them, you will have a tough time with Diablo.
- Demon lords have 70% fire resist in Normal. So if you are using Holy Fire, then you need to use a different attack.