So, I've got a faction I'm developing for my homebrew, based around a sort of assimilating force that's attempting to devour the world.
Therefore, I'm building a template to how assimilated creatures are changed, listed in the spoiler below.
“Abyssal Creature” is a natural or acquired template that can be applied to any creature (referred to hereafter as the base creature). An Abyssal Creature retains all the base creature’s statistics and special abilities except as noted here.
CR: ???
Alignment: Abyssal Creatures are treated as if they possess the Beyond Morality Mythic Ability, though this does not give them Mythic Ranks.
Type: The creature’s type changes to outsider. Do not recalculate BaB, saves, or skill ranks.
Senses: The creature gains one of the following: Darkvision 60 ft., Tremorsense 30 ft., Blindsense 30 ft., Blindsight 10 ft., Lifesight 10 ft. If the base creature possessing the chosen sense, it’s range is extended by the listed amount.
Size: The creature can shift one size category larger or smaller, chosen when the template is applied.
Equipment: If the creature is listed with any equipment, it is assimilated into the creature’s being. Armor bonuses become Natural Armor bonuses, magic items continue to provide their bonuses, and manufactured weapons gain the benefit of immunity to Sunder, as they become integrated into the creature’s form. This reduces the “treasure” entry to 0 gp.
Defensive Abilities: Abyssal Creatures, thanks to their alien nature, are granted immunities to certain forces, especially those related to the standard nature of the universe.
Immune to Fear, Critical Hits, Precision Damage, Positive and Negative Energy, Soul Magic.
Abilities: +4 to ability score of choice, and +2 to all others.
Feats: The creature gains two Teamwork Feats of it’s choice.
Special Qualities: The Abyss grants the following qualities:
Hivemind: The creature is linked to the Abyssal Hivemind, operating as per Telepathy. This grants the creature as having any Teamwork Feats its allies possess, however it does not gain the benefits of those feats. The Hivemind is completely aware of all the creature does. If the creature is severed from the Hivemind, the creature is stunned for one round.
Easily +2 CR without a doubt, and the template looks like something you'd see for vampires, graveknights and liches, which means the base creature needs to have at least 5 HD before the template is added. It may even be a +3 CR.
Yeah, I see this being CR +3. The ability score increase is slightly more than the Advanced Template, and the size change definitely makes that more akin to CR +1 for that section (since it will always be an advantage for the creature in question). All the others abilities are probably another CR +1, maybe +2 if you really push it.