He is an alternate iteration of Primate Murder -- inhabiting those timelines wherein the Common Sense of Man pulses with life, and the Twenty-Seven Dead Apostle Ancestors do not exist as an organization. He is known as Beast IV.
Originally a Beast of the Planet that became Merlin's familiar, Cath Palug was at some point subsequent to Merlin's sealing driven from the Isle of Avalon. In those worlds where the Common Sense of Man is insufficiently prominent, he came eventually to be known as Primate Murder -- the pet of Altrouge Brunestud.
In the timeline of Fate/Grand Order, he came to reside within the Chaldea Security Organization, journeying across the assorted eras of human history in the company of Ritsuka Fujimaru and Mash Kyrielight.
Cath Palug describes himself as the Beast of Calamity (災厄の獣, Saiyaku no Juu?) that bears the Characteristic (特徴, tokuchou?, lit. "distinct feature") of "ever exceeding the strength of an opponent" (相手より強くなる, aite yori tsuyokunaru?) -- growing unto maturation by the conflict that arises amongst Men, feeding upon their Envies (妬み, Netami?) and Regrets (悔しさ, Kuyashisa?).
Mash Kyrielight describes the Cath Palug as a squirrel-like creature -- "a cute animal," according to his official profile. However, he states that if he were let to normally mature, he would eventually degenerate into a "unsightly monstrosity."
Removed of the Civilization of Man, Cath Palug would be nothing more than a harmless animal.
He considers Merlin to be a "loathsome wretch," claiming that the man forced him from comfort of the Tower of Imprisonment (幽閉塔, Yuuhei-Tou?) in Avalon against his will. As such, he frequently attacks Merlin on sight (illustrated in Merlin's card for the April Fool's 2017 event, giving Merlin a flying kick to the face).
Garden of AvalonEdit
Upon witnessing Arthur's final moments, Merlin releases Cath Palug from the Garden of Avalon, insisting that he seek out "that which is beautiful."
Cath Palug is found by the Chaldea Security Organization and given the nickname of "Fou-kun" (フォウくん, ?). During his stay with the organization, he is permitted to freely wander their facilities.
Fuyuki: Contaminated City in FlamesEdit
E Pluribus Unum: Grand Battle of Legends in North AmericaEdit
Merlin uses Cath Palug as a conduit through which to temporarily reveal himself, so to lend aid to the protagonists in the battle against Cú Chulainn Alter.
Salomon: The Grand Time TempleEdit
In the aftermath of Mash Kyrielight's sacrifice, Cath Palug locates her soul in the place beyond life -- revealing his true name and his nature as a Beast. He then expends the majority of the Magical Energy that he's collected across the centuries of his life to transplant his Force of Providence into Mashu's soul, perfectly resurrecting her. However, the act deprives him of both his Sapience (知性, Chisei?) and Characteristic (特性, Tokusei?), leaving him as nothing more than a common animal. In the end, he congratulates Mashu and the people of Chaldea for defeating the Beast IV without violence.
Under certain circumstances, Cath Palug is capable of enacting a miracle that exceeds the scope of True Magic -- "the Complete Revocation of Death" (死者の完全な蘇生, Shisha no Kanzen-na Sosei?). More specifically, he was able to resuscitate Mash Kyrielight, but this was possible only due to the extra-temporal nature of the context of her death, the interior of Ars Paulina.
So to facilitate the process, Cath Palug consumed the majority of the Magical Energy he had accumulated across the centuries of his lifetime, using it to fuel the transference his Force of Providence to Mash, and overwriting the limitations of her remaining lifespan (3 days). However, as he wasn't yet an adult of his species, he was capable only of granting Mashu a lifespan comparable to that of a normal human -- at the cost of his own Sapience (知性, Chisei?) and his Characteristic (特性, Tokusei?) as a Beast of Calamity.
Taiki is the illustrator for Cath Palug.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
 Fate/Grand Order, Final Singularity, Chapter 15 - Beast IV
- ↑ 2.0 2.1 Garden of Avalon - Chapter 06: In the Flower Garden, p.103
- ↑ 3.0 3.1 3.2 Fate/Grand Order - Fuyuki: Contaminated City in Flames - Prologue
- ↑ Fate/Grand Order website: Fou: A cute animal who the protagonist meets alongside Mashu. It seems to walk around Chaldea freely.
- ↑ Garden of Avalon - Chapter 00: At the Garden, p.013