Hidden area

Locate the Tower Of The Magi, in the south-east area. Enter Eas Ter Egg as coordinates in the teleporter room. Enter the cave in the new area and step on the rune. Defeat the enemies that appear, then step in the various runes in the new area to see the game's developers.

Change random shop items

Press [Shift] + B while outside the shop window to change the random items that appear in the shops.

New hostile monster

When a splitting enemy that has been summoned by Simulacrum is hit and splits, the new monster will be hostile. Note: This was tried with DoomGuard.