Garlean Inability to use magic.

[rainichan]

As far as Emet-Selch goes, he cheats and can cast magic on the Source due to the benefit of being an Ascian. He is basically a ghost of an Ancient possessing the body of a Garlean who was once his own person and had no capability to cast magic on his own. Whenever we interact with Emet-Selch on the First, he is not actually in the body of an Garlean, he possessed some random person and then molded his body to look like Solus because that's a form he's comfortable with.

He had clone bodies made for him, he mentions this when he's first introduced. He has many so he doesn't have to possess random people like Lahabrea did, which he takes umbrage with and mentions it "diminished" his ability and made him weaker: "According to Emet-Selch, Lahabrea had a habit of discarding host bodies when they were no longer useful, weakening himself and allowing King Thordan to absorb his essence." Having a body he already knows and likes is why he had the clones made and moving his soul into them and from a post from Anonymoose: "The clones were, according to Emet-Selch, just to save him the time and energy of molding a physical body to his preferred image." The clones were destroyed later by Estinien and Gaius iirc when they infiltrated the Capital, but by then Emet had his body and traveled to the First.

But since he is indeed an Ascian, he can cheat and use his own abilities even if he's in a cloned body of Solus.