This creature - the Apostle of Immorality (AOI) is the only means to spawn the Apostle of Malevolence (AOM):
https://bdocodex.com/th/npc/21979/

There are a few theories running around, as yet unconfirmed:
1) you have some chance (dictated by an RNG) to get them (AOM) to spawn on each kill of an AOI
2) kill a bunch of Apostle of Immortality, the more kills the better
3) kill all the Stars End mobs as fast as possible, the more kills the better
4) kill Apostle of Immortality near the Unstable Star Debris, specifically by using it to knock them down; then down-attack them on the ground

The only thing I can confirm, is that you only get Apostle of Malevolence to spawn after an Apostle of Immortality dies.