you can make a procedure with the global trigger of entity spawned in if the entity is a pillager summon (custom mob) with this, when a pillager spawns in a raid, it will spawn in with the custom mob if you don't want the pillager there, you can use the despawn event/target entity block
If you go to the top and find the tag button, you can add a tag into your workspace called "minecraft:raiders", and under that, add your mob.
Sadly didn't work got any other ideas?
My next tutorial is custom raids.
example
Minecraft Custom Raids - YouTube
Use command block for raids.
Chew
you can make a procedure with the global trigger of entity spawned in
if the entity is a pillager
summon (custom mob)
with this, when a pillager spawns in a raid, it will spawn in with the custom mob
if you don't want the pillager there, you can use the despawn event/target entity block
denial719 that's not how you do it. All your doing is spawning an custom mob that's not part of the raid.