Unable to continue in 'defeat the united one' quest

[Pinafal]

When getting the quest to discover a spell that Elpiritster the Eternal doesn't know yet, i went through the options to save the witch and payed her 1000 gold.

I received the seasoned tax collector but it didn't finish the quest, it just simply made it disappear meaning i'm still having all my land terraformed. I'm assuming it's due to it not actually being a spell i can cast?

 

[WingedKagouti]

When you pay the witch you're supposed to get a new quest (find and raid a specified monster lair), which will give you a spell you will need to cast when you complete that.

 

[Pinafal]

The only quest i have is the main quest. And the spell is a perk that's already given to me.

 

[WingedKagouti]

The only quest i have is the main quest. And the spell is a perk that's already given to me.

Raiding the indicated lair should have given you a summoning spell (Rainbow Dragon) which you would then be instructed to cast. Are you using any kind of mods?