To clarify. Initially I was going to ignore 2 Liches after entering the level so I stepped 1 tile to west. One lich summoned Shadow Fiend, another Lich summoned Hellion. I moved back to stair, got hellfired and one of the liches summoned another Shadow Fiend. I escaped upstairs of course.
Next time I got Executioner and Green Death so I retreated again.
Next time I was luckier and could move out of LoS without any summons.
After clearing Depths 5 I hasted out of LoS and was going to kill them all but then there were 4 Liches inside the vault
There was a permanent Rakshasa inside the ziggurat vault (it was teleported there I guess) and it cloned Lich before my first action in their LoS.
Any way I digged a killing hole and killed them all after disintegrating the wall. 4 Liches is surely lots of fun