Originally Posted by
TresserT
My take is pretty much this:
CHALDEAS is a planet in and of itself (stated to be so by Daybit). As we know, planets have wills. The "Alien God" is the will of CHALDEAS.
Marisbury's plan is to world egg the entire universe into CHALDEAS, then hatch it and reach the Root in the process. The fact that this would destroy the universe, Earth aside, is a necessary sacrifice. (This is also why the stars have vanished. The cosmos is currently inside CHALDEAS)
Olga has the power to make simulations into reality, as well as to end them (see: MBBAAN). This was her purpose as designated by Marisbury.
The will of CHALDEAS doesn't want the universe to end. After getting its hands on an asset like Olga (by Olga being thrown into it by Lev), it summons her as a Servant, a protector of CHALDEAS, as well as the other 6 Apostles for the purpose of preventing Marisbury's plan from coming to fruition. Olga revives the 7 Crypters by using her simulation>reality ability. She freezes CHALDEAS so that it won't hatch. She makes the Lostbelts for the purpose of getting herself a body on Earth rather than being confined to CHALDEAS. Once she's on Earth, she wants to make a better future for humanity.
Chaldea tears up all of her plans, thus letting Marisbury's plan proceed without interference any longer.