D&D 5E You get one spell vs. a demon lord... which one?

[Eltab]

Scroll of Summon Tarrasque

The demon lord possesses the body of the Tarrasque ... that did not work out quite how we had planned.

 

[Oofta]

The demon lord possesses the body of the Tarrasque ... that did not work out quite how we had planned.

Or the demon Lord looks at the tarrasque and says "Bob? Is that you? OMG! It's been ages! I mean when was the last time we got together for a good city stomping? What say we take care of these pests and have a night on the town for old times sake!"