Ancients drop random items on death.
This "tower", aka Infernal Contraption, is the last wave on Baal Run. Except that it actually deals damage, and tanks quite a lot, it keeps spawning random minions. If you don't kill it quickly, you can find yourself in a quite tricky situation with Throne Room being packed.
After killing that last wave, Tran Athulua portal opens. First uber level.
note: you must be between lvl 50 and 59 to do this quest. If you're higher than lvl 59, you must pay 1 Arcane Crystal per 10 levels to get Charm from this level.