Or it could just be some scrapped ideas [spoiler]but that'd be really cool if it was implanted in HoW[/spoiler]

Have you guys seen servitors and fallen together sometimes? The servitors cover the fallen in some kind of purple aura, I've not shot them in this state, are they invulnerable when covered by that?

The Servitors are Fallen units. It's still not clear how they're related, unless it's in a Grimoire card. Anyway, Servitors are often majors (yellow health bar), signifying them as special units. If you run The Archive mission, you'll run into a Servitor major, some Vandal majors, and mobs. The Vandals tend to stick by the Servitors as well, which could indicate deference. That purple stuff? The Servitors can heal Fallen, using void magic I guess. It's slow, so you may not have noticed if you were going about the usual alien genocide. The weird thing is that Sepiks Prime is worshipped as a God by the Fallen, but there's no explanation for that. I just assumed the Fallen are impressed by physical size and this manifests itself in their social hierarchy. "Hey, Draksis, you're tall as shit! Think you wanna be a Kell?"

Servitors are a technological remnant of when the Fallen were a mighty empire. There was a war and the fallen basically destroyed themselves. Prime Servitors are the brains were as the small servitors are the phalanges. Prime servitors are said to have a tight symbiotic relationship with the fallen Kell. They do heal and shield nearby allys except perhaps the Primes lack this ability as Sepiks Prime does not do this.

Is that from the Servitor Grimoire?

Yup, except the healing and shield part.

Yea I've noticed that too, never knew what it did though I guess servitors are a support unit providing buffs and stuff to their fallen buddies