When you zone in, there's nothing but some non-agro trash mobs. You can ignore the rats, but it's safer to kill the spiders so they don't get in your way later and you can get some AA for killing one.
Near the entrance, there are 6 graves. If you hover the mouse over them you get the name of the hero buried there, and you right click on them, you're given an option to either whisper a prayer, or defile the grave. Defiling a grave causes A desecrated remains to spawn, which must then be killed. Either way, you must interact with all of them. One of the six is Lord Tonmerk's.
After you've interacted with all 6 graves, walk into the second room and wander around until Ronicus the Red and his crew spawn.
Now, depending on whether you prayed or defiled the graves, Lord Tonmerk will spawn, either as an NPC or a 22^^^ paladin. Praying at all the graves causes him to spawn as an NPC, and if you talk to him, he'll give you an Idol of the Eternal. If you defile any of the graves, he spawns as a mob instead and drops Idol of the Wretched.
Then click the door behind him to leave the Sepulcher.