To pre-emptively answer your question:

"[Irish mythological hero/demigod Cú Chulainn] is known for his terrifying battle frenzy, or ríastrad (translated by Thomas Kinsella as "warp spasm" and by Ciarán Carson as "torque"), in which he becomes an unrecognisable monster who knows neither friend nor foe."

I.e.: Hulking out.

(Merely here due to Irish mythology? Or also the influence of Marvel Champions?)

from The Táin (Táin Bó Cúailnge) [translation KInsella]: "The first warp-spasm seized Cúchulainn, and made him into a monstrous thing, hideous and shapeless, unheard of. His shanks and his joints, every knuckle and angle and organ from head to foot, shook like a tree in the flood or a reed in the stream. His body made a furious twist inside his skin, so that his feet and shins switched to the rear and his heels and calves switched to the front... On his head the temple-sinews stretched to the nape of his neck, each mighty, immense, measureless knob as big as the head of a month-old child... he sucked one eye so deep into his head that a wild crane couldn't probe it onto his cheek out of the depths of his skull; the other eye fell out along his cheek. His mouth weirdly distorted: his cheek peeled back from his jaws until the gullet appeared, his lungs and his liver flapped in his mouth and throat, his lower jaw struck the upper a lion-killing blow, and fiery flakes large as a ram's fleece reached his mouth from his throat... The hair of his head twisted like the tangle of a red thornbush stuck in a gap; if a royal apple tree with all its kingly fruit were shaken above him, scarce an apple would reach the ground but each would be spiked on a bristle of his hair as it stood up on his scalp with rage."

Eldritch horror indeed.

(Honestly, I'm disappointed we didn't get some flavor text excerpting a translation.)

Sadly, this card is mostly useless at the moment. I played it through a campaign as William Yorrick with a chainsaw - which seemed like about the best-case scenario for dealing the excess damage. But even after playing it many many times, I still almost never managed to get a single resource on the card.

In order to deal even 1 excess damage, you usually need to spend something that's worth more than 1 resource - like commiting a Vicious Blow, or wasting precious Chainsaw ammo on a weak enemy. The reward just isn't worth the cost, especially when you compare it to Emergency Cache.

All hope is not lost, though. I personally hope that we will see more cards to support this "Overkill" archetype in future. With more ways to overkill enemies, and more rewards for doing so, this could become a really fun playstyle. Perhaps someday there could even be a guardian who focuses entirely on overkilling enemies for big rewards. This could eventually redeem this card, in the same way that Winifred Habbamock finally redeemed Opportunist.

If that day ever comes, I will be incredibly excited to take this card out of the binder.

Contrary to the other reviews here, I do think this card is quite good.

There are maybe ~18 rounds average on the doom clock in Arkham.

• If your investigator has soak/healing (Tommy, Carolyn, etc) you can use it to straight cancel 2-3 of the worst treacheries you draw (~1/6th of your total in a game!), and transfer some more into random ones (using the 'remove corruption' option, depending on just how much soak/heal you have)
• If your investigator doesn't have soak/healing, you can still use it to cancel 3 bad treacheries and take a random one later (in any fast action window!)
• It makes you more resilient to unfortunate game states, and allows you to maximise your chances:
  • You pull something and have no skill cards in hand (wait until you have a hand, then draw one & heal)
  • You can choose to pull the 'instead' encounter card(s) when grouped up with the team so they can commit if necessary
• You can effectively use it to transfer damage/horror between your investigator card and your allies
• It can save your life — there'll be a fair few occasions when a treachery you draw will do worse than 1-1 to you
• It can buy you extra rounds of the game, in scenarios where Ancient Evils and other 'add doom/can cause agenda to advance' effects are in the encounter deck
• Calvin can use it to take damage/horror quickly and then heal a bit for a fast action when necessary (bonus: I believe the treachery you draw will be tested at your higher stats - i.e. before healing - because it's part of the cost)

TL;DR is bad encounter deck pulls at the wrong time is the easiest way to tank a scenario (possibly the only way for a lot of us), and something most investigators don't really have much say over... at least until now (neutral, asset with uses). Straight cancelling is super powerful if you can afford the cost here, and if not then transferring a bad treachery for a random one at a time of your choosing is also strong. It's expensive, but worth a look.

When replaying Point of No Return, it is absolutely vital to have a plan for dealing with this location (unless you're fine with accepting R2/no resolution) -- it is a mandatory gate for advancing Act 2. Although there are tons of clues available in the other locations, you cannot actually flip Vale of Pnath unless there are specifically no clues on IT, and hitting Shroud 4 without committing or playing cards can leave even dedicated clue-getters subject to the whims of a small number of tokens in the bag unless you can rely on multiple static boosts and/or triggered abilities. For many solo investigators it will be flat-out impossible unless specifically built-around.

(FWIW, in my personal games, I house-rule that Vale of Pnath's veiled ability can be triggered as if there were no clues remaining on it if the investigators have "surveyed the vale" -- with flipping Crag of the Ghouls or Peaks of Thok leading to remembering that the investigators have "surveyed the vale." I think that offers a more varied way for investigators to contribute to advancing the act.)

It's a nice touch that the Ghouls in A Thousand Shapes of Horror spawn in the graveyard. Dream-Quest of Unknown Kadath makes it pretty clear that the "ghouls of the waking world" actually live in the Underworld, and simply cross over from the Dreamlands to feast in waking-world graveyards. So the Ghouls are only here due to random proximity to the Underworld entrance, because they're coming up from the Dreamlands' Underworld for their snack time, unrelated to the Unnamable itself.

Similar to the Web of Dreams overarching story, I suspect that what happens to your house in The Gathering is also probably a Dreamlands/Underworld-merging-with-the-waking-world thing, even if it's never explained in Night of the Zealot.

(Mechanically, it's also a way to make area-of-effect damage more relevant beyond just the swarming mechanic (in fact, probably more so, since most swarms are more easily dealt with via normal attacks/evades).)