The quick snarky answer? They're sourcerors.
The longer answer? Right, so, source as a gameplay mechanic in D:OS2 is not what source is as an aspect of the lore. From what the OS series has established source is a special kind of magic that works on different principles than "normal" magic, and is drawn from a completely different source of power. In D:OS2 source is like the force in Star Wars, it's everywhere and in everyone and it congregates in various amounts. Sourcerors can tap into this power and unleash absolutely terrifying magical powers beyond the scope of simply throwing fireballs. Magic turns you into a chicken, source turns you into a burning pig for all eternity which requires divine intervention to undo.
So the reason they're a big deal is that source is functionally absolute power; and absolute power corrupts absolutely.
