Souls' (Emet-Selch's) lineage is implied to have inherited some measure of his power with the size to match.

Beyond that... it's unclear. It's not so subtly implied by Elidibus and Fandaniel that Emet-Selch experimented on him in some manner, and he claims to have had dreams of the Final Days as long as he can remember. The latter implies he may have been a reincarnated Ancient, but then there's the fact he needed Resonance experiments performed on him to gain access to the Echo...