Quick debrief for those who aren't familiar with the characters. Warb Null was a heavily armored brute with a physicality that could be equal to if not above the level of someone like Savage Opress. He was a mindless sith warrior who sported special ancient sith armor and wielded an atypically large lightsaber with a hilt several times the length and width normally seen. He was said to wield the blade with skillful aggression, on top of this he was said to have learned certain 'dark powers' which could refer to force powers.
Zalatosh Kavros was a Jedi Master who fell to the dark side serving under Revan and Malaks Sith empire. What put Zalatosh on the map in terms of power was when Lord Kallig AKA Darth Nox searched for Zavros as well as other 'legends' in order to bolster his own powers by absorbing their respective force ghosts. I suggest this means he was of worthy calibre since when Nox added said ghost as well as other legendary spirits his power grew by several magnitudes; enough that it meant he could face his rival Darth Thanaton.
Kalatosh as of when he was alive and in-the-flesh.
Round 1 - Duel
Round 2 - Force
Round 3 - All out