About Grand Berserker have we atleast the Quote of Daybits servant and i am honest my Grand Assassin candidate is Fuma Kotaro the first one .I had a line of Indrajit as Grand Assassin (my idea being that simply being a killer or strong isn't enough one needs a special kind of connection with death) using the idea of him having access to the powers of the Trimūrti (albeit I had tobang my head against the wall to find accurate information on if he did have all 3 Astras of the Trimūrti, consider scrapping the sheet when I found that there was no definitive proof he had the ultimate Astras of either Shiva or Vishnu, then explain why he had Shiva's astra (easy) and how he had Vishnu's (harder but I managed)get a little creative with it, but I managed) and heightened perspective associated with being a demon (a higher class of being that the of a human) to understand the nature of the Trimūrti as the 3 aspects created by Brahman the ultimate truth of the universe and basically the force which fuels all gods [and within my head canon is a pathway to the root in it's own right]. One connected to Brahman who understands Brahman basically gains the power of Brahman, essentially making them a god in that respect, in my head canon this is how Buddha escaped the cycle of reincarnation.
While Lore-wise I'm going with the idea that Indrajit never used Trimūrti-astra (the name he gave to the hypothetical technique of using the power of Brahman by combining the 3 ultimate Astras together as an offensive weapon) partly because he felt it morally wrong to use it as a weapon, partly because he feared reprisal for doing such from the Trimūrti and partly because it sacred him (to give an idea of what this could do, hypothetically take Hanuman the strongest warrior in the war between Rama and Ravana, if this were used on him, 1 hit would be enough that it would cause his soul to basically try to escape this mortal coil and then transcend the physical universe, forcefully pulling him from the cycle of reincarnation by bestowing upon him the ultimate truth [Hanuman could try to resist this, but it would basically take all his might to do so, to the degree that basically a single good hit after that would distinct him and then cause his soul to escape his metaphorical grasp]) just how strong it was. As a grand I imagine similar to how King Hassan bestowed the concept of death on Tiamat it could "place or removing things from cycle of reincarnation" thus allowing beings like Tiamat to be killed, and things which should be invisible to be destroyed (or even straight up removing things from existence)
I'm not sure who exactly mentioned it but back when I was doing my lostbelt I did mention the possibility of Cain as a Grand Berserker* or Grand Assassin** (and even had him appear in early drafts [the version of him I had probably was wank enough to count as a grand servant]). As for Wendigo I'm not sure it will work either, but I suppose I just will have to try or keep looking. I do want to eventually try to and get some degree of coverage for the entire globe [save Antartica for obvious reasons] at least as a proof of concept that Grands don't have to be restricted to just Europe or Asia (I was also throwing around the idea of Walumbe / Warlumbe (if he would even count) as an African Grand servant (not from Egypt) as Grand Assassin (he was cited to be the cause for all disease and death in Gandan myth [I just may want to wait on this one as the subject of disease isn't a topic I'm comfortable doing a sheet based around at this point]).
* While never explicitly going insane the sheer pointlessness of his murder and the way he is viewed in modern culture could certainly lean in this direction
** Yet again while certainly not stealthy Cain's actions constitute the 1st instance of someone having ever died in the Aberhamic myths (before even Adam or Eve died) and as a result basically unleashed death onto the world, this combined with the fame of the act would make him probably debate as fitting the role in this point of view.
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As for what I quantify as Grand Berserker, my current definition is that being insane or having gone insane isn't enough (aside from the given that you have to be powerful), rather similar to Grand Saber, you have to basically either exist in a state of perpetual insanity (Sisyphus) or be basically what amounts to the embodiment of the concept of insanity (Jack the Ripper [from Strange Fake]).