mechanics analysis:

0:09: Cyclone-like attack hits multiple monsters, each one has a blue flash at the head. Only one keeps the blue and thus gets a blue head.

0:10: he does a quick attack (with blue effects around his hand) and kills the blue head monster. he gains a blue charge.

0:14: he does some sort of elemental attack. the attack has a blue effect & he loses the blue charge. blue charge presumably spent to power the attack.

0:15: each monster he hit with that attack gains a blue head, but when he kills them with normal attacks, the blue charges disappear.

0:18: second attack on big monster gives it a blue charge.

0:19: he does a quick dash attack with blue effects, killing the big monster & gaining a blue charge.

0:22: expends another blue charge to do big attack proceeds to do cyclone-like attack, giving another monster a blue head. he kills it with another blue dash & gains a blue charge.

summary:

new blue charge mechanic. seems to work like this:

first, you get %chance to (give monsters blue heads) on hit. seems to be global, (not skill-specific) but may come from specific supports. you can see its not guarenteed as the big monster takes two hits to get a blue charge.

second, you use a special blue charge-stealing skill. there is a dash and a close-up grab. perhaps they are the same skill, just adapted at different ranges.

third, you expend the blue charge to unleash a powerful attack.

edit: calling it as marauder variant. i dont know why, he just gives off marauder vibes. also, the charge has a duration, as it runs out during the boss's introduction