"If some of them killed your people it's right to exterminate all of them?" Ghiso unsheated his cold sword "Not that it matters at this point"

Advancing to the nearest cultist, he cut into them with the sword.
"I don't know your reasons, but are you really willing to throw your lives away for no reason? You aren't defending something, nor do you gain from our death"

Spoiler: Ooc
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Ghiso will advance to the nearest damaged cultist and use Booming Blade, hopefully stopping that one from moving past him.

To hit (1d20+5)[6]+(1d4)[2]
(1d8+5)[12] magical slashing+(1d6)[4] cold damage

Bonus action... Not really.

If hit by an attack or if he fails a saving throw by little enough he'll use Arcane Deflection (either +2 to AC or +4 to a save) as a reaction.