You're not alone ( ≧Д≦)
I’m still trying to understand how the hell Seth was powerless at one point and now he has his powers back like WHAT?
He's borrowing the cursed souls' temporary support, so maybe he's given access to the power of sand for as long as he has the souls' active support. The souls are probably agreeing to temporarily support him, and even lend him extra power, because they've seen he's truthfully repenting, and also that he's using that power to punish the humans that have been harming Egypt.
Furthermore, another point that I just realized, that makes it more clear that the reason he can use his power now is because the souls are now actively helping him, is that the weapon Seth's summoned this latest chapter is probably working for him now, because it is the very same one that was MADE with those souls: it's the same cursed sword that Seth said he had created by destroying those particular human souls (which he'd done in order to have an effective weapon against the undead Osiris).
I'm about to take my test in 5 mins yet reading your explanation instead of revisioning my studies. Anyway thanks!
Hahaha, you're welcome, wish you lots of success in your test!
My silly little brain struggling to understand the plot