The secondary personality of Jekyll And Hyde (Fate) that manifests after Henry Jekyll (Fate) uses his Noble Phantasm (Fate) Dangerous Game.
In Fate/Prototype: Fragments Of Blue And Silver, the transformation results in him appearing as a Werewolf-like creature. After being corrupted by the Holy Grail (Fate), he becomes the dominant personality and can temporarily maintain a human form for only an hour at a time.
In Fate/Grand Order, the transformation keeps him in a human form, but he switches his class from Assassin to Berserker.