Blood Moon Ari is one of the Monster Girls from the game In Heat. Blood Moon Ari is the alter-ego/transformation of Ari that only appears when she's affected by a particular full moon.

The only way the player can see Blood Moon Ari in game is by doing the following:

Nightclub: If the player pairs Ari with a client in the Diamond Room, Ari will transform and kill the client, rendering the room and Ari unavailable for the remainder of the night.
Security Office: By surviving to Night 12, Blood Moon Ari will appear as the modes final boss. The only way to stop her from murdering the player is to observe her via the cameras, as she doesn't move when being watched.

Blood Moon Ari is voiced by JellyfishJubilee: https://x.com/JellyfishJubi