TMdict

Arcueid after she has succumbed to her vampiric impulse and lost her ability to reason.
Though regarded as equivalent to the Crimson Moon, she is a completely different entity. You could say she’s the materialized form of Arcueid’s overflowing vampiric impulse. When Berserk Arcueid appears, Arcueid herself is freed from her desire to drink blood.
…That’s a complete and utter lie, of course.
Unlike Arcueid, she enjoys trampling on her enemies with overwhelming force. Because as a True Ancestor Arcueid does not (cannot) do useless things, she always regulates her fighting ability to match her opponent’s. However, Berserk Arcueid will use her power mercilessly even against normal humans.
Incidentally, her over the top speech and behavior is really amusing.