I see the problem here. Previous editions often gave monsters (particularly planar monsters) a whole bunch of special abilities that were either flavor or downtime stuff – often in the form of a laundry list of spell-like abilities. For example, in 2e a marilith could cast animate dead, cause serious wounds (reverse of cure serious wounds), cloudkill, comprehend languages, curse (reverse of bless), detect evil, detect magic, detect invisibility, polymorph self (7 times per day), project image, pyrotechnics, and telekinesis in addition to the common tanar'ri spell-like abilities of darkness 15' radius, infravision, and teleport without error. 3.5e cut down a lot on these in favor of combat-relevant abilities, but not completely – the 3.5e list is align weapon, blade barrier (DC 23), magic weapon, project image (DC 23), see invisibility, telekinesis (DC 22), greater teleport (self plus 50 pounds of objects only), unholy aura (DC 25). And 4e went even further with turning everything into a distinct Power.
The reasoning here is that the marilith is one of the most powerful non-unique monsters in the MM, and very much an endgame monster (in 3.5e it is CR 17). Things like animate dead, cloudkill, cause serious wounds, curse, and pyrotechnics are kinda useless at those levels, so off they go. But those abilities could still be useful off-screen in the proper circumstances – perhaps they used cloudkill to murder off a whole orc tribe and then animate their shambling corpses and set them on nearby tribes – that's flavorful, even if it's not something that's relevant to level 15+ characters and thus shouldn't take up space in their stat block. But it could still be part of their lore, perhaps in the form of 4e rituals?