Two kittens are fighting very fiercely.

Kittens frequently play rough, which may appear to be fighting but is actually a normal stage of their growth. Playfighting aids in the development of critical social and motor abilities. To make sure that their behavior doesn't turn into actual hostility, it is crucial to keep an eye on it.
In the event that the play appears unduly harsh or if you are worried about their safety, you can take the following actions:
Watch Their Body Language: If they are hissing, have flattened ears, or have hair standing on end, it could mean that the play is becoming aggressive. It would be wise to step in if this is the situation.
Offer Diversions: To divert their focus, toss in a plush toy or present a novel plaything.