Click the totems in the middle of each room. Most of the time they will do nothing, but occasionally (once every couple of days or so) they will come active, then clicking them will spawn a Tomb Guardian and a bunch of mobs. Be careful because they spawn right on top of you. TG are similar in difficulty to biles but are a little bit faster and have slightly increased stats and about double the HP. When all 4 guardians are dead a timer will start that spawn lord of the dead. Lord sometimes spawn instantly, sometimes it's delayed up to an hour. There are a few specific places it can spawn in the main maps, and there's a global announcement, so you godda be fast before pk comes and steals it