Presence is a female voice from a sith holocron that tried to lure Ezra Bridger to the dark side in Star Wars Rebels.
Ezra first spoke with Presence when he found the sith holocron on Malachor in the episode “Twilight of the Apprentice.”
Ezra speaks to her again in the episode “Steps Into Shadow.”
Dave Filoni spoke about Presence in this interview with IGN. The entire interview is great but here are a few interesting snippets about Presence from Filoni:
To me, that voice you hear is actually the ancient, ancient Sith Lord that created the temple. She was a powerful Sith Lord. I like the idea of this powerful, ancient woman that has domination that we’ve never heard of before, that was creating this massive weapon.
But yeah, there’s this ancient Sith Lord that Nika was playing, and I’m still going over the dos and don’ts of the continuity of all that. But I wrote the history down, I just put it out there in case people want to pick it up later when they’re working on things, and give some continuity. I won’t reveal the name – I have named the character, but I have to make sure everybody’s cool with it.
Filoni also talks about Presence around the 23rd minute of this podcast interview from Geeky Bubble.