I'd note that if the spell does electricity damage, it should have the [Electricity] tag.

I'd disallow this spell in my game for using the conjuration spell exploit to get it unaffected by spell resistance. It's even worse when the spell clearly is based off Flamestrike, and that is an Evocation that allows for spell resistance.

And then to make it really unbalanced, this spell can charge up a melee weapon too?

A good way to tell if a spell is broken is to ask the simple question of ''what would I say if this spell was used against my character?" Would you be all right with the normal damage an attack does, plus 5d8 more damage? Every round?

Also the spell is not really ''thunder'', it's ''lightning''....