Apostate Tower appears in the Harabah Block of Tartarus, from floors 165 to 179. It will generally appear in groups with other shadows but will also come in groups as large as four, which will make for a larger EXP yield before party members become tired. They will mostly use their poison skills with occasional electricity or piercing attacks.
Apostate Tower appears in Paradises 3 and 4 of Heaven within the Midnight Channel. It will use Electricity spells to target the party. Upon being defeated, it may drop Sacrilege Iron. Selling 8 Sacrilege Irons will unlock the Number One weapon for the protagonist in the shop.
In Golden, it appears in Paradises 1 to 4. It may drop Meteorite for trade at Shiroku Pub.