I reached out to FFG through their form regarding this card and got the following, the latter half being specific to Soul Sanctification.

Q: Can you play a card or use an action to heal while at full health/sanity? I.e. Playing Soothing Melody to draw a card while at full health/sanity.

A: No, you cannot normally heal a card that is at full health/sanity.

Q: While you have Soul Sanctification in play, can you play a card or use an action to heal your investigator while your investigator is at full health/sanity?  For example, can I play Emergency Aid on my full health investigator to generate two resources on Soul Sanctification?

A: Yes—Soul Sanctification makes a specific exception to the rule I stated previously. With Soul Sanctification, you are able to "heal” your investigator even if you are at full health/sanity, and place any excess points healed onto Soul Sanctification as an offering. Emergency Aid would place 2 offerings on SS.

Updated for the Scarlet Keys.

This is the only card that I have found that can reduce a location's shroud outside of an investigation test. This opens up cool interaction with two interesting new cards : Power Word and Runic Axe, specifically with the "Confess" and "Elders" upgrade. Both allow you to automatically discover clues, but depend on the shroud of the location, which you can lower via Arcane Insight.

Who could use this / be interested :

For the Arcane Insight + Power Word combo :

-Daisy Walker and Mandy Thompson have access to both (limiting their Power Word to level 2 and 1, respectively), but probably have better ways of getting clues.

-Marie Lambeau has access to Arcane Insight as a spell, can upgrade Power Word to the max and can also take advantage of the lowered shroud by investigating normally. She can also use her extra Spell action on the Power Word parley.

-Akachi Onyele has access to Arcane Insight since it uses charges, gets an extra charge and can upgrade Power Word to the max.

For the Arcane Insight + Runic Axe combo ("Can I axe you a question ?") :

-Akachi again has access to Arcane Insight and Runic Axe because charges, can upgrade the axe to level 4, and has a surprisingly decent Fight.

-Ursula Downs could take the Runic Axe as a Relic, but her low Fight would make it difficult to wield.

-Joe Diamond can upgrade the axe to level 2, and it plays into his strength as a flex fighter/cluever (perhaps for solo ?)

Finally, since you can use this card during another investigator's turn, any Seeker (except Vincent Lee) could do these combos with their Mystic or Guardian buddies.

Notably one of the only customizables Charlie can take, and one that has a couple of uses for him. Dominance (2XP) gives him a handy extra ally at less XP cost than a Charisma, Dreaming Call (3 XP) gives him the ability to bounce back Art Students or damaged allies, Wings of Night (1 xp) gives him a ride and potentially a taxi service, and any of the others gives him a bonus action at 4+1 wild for exhausting, which is both a substantial buff and a mini-Leo.

With all of those, you have a 7 XP bonus ally you can move to and leave at hard to stomach locations and still exhaust in mythos for 2 will or investigation phase for 1 ?. Not terrible for late game XP soaks, but probably not the first XP you're spending. For decks that want 4 Summoned Hounds, or just can't deal with that cards more considerable drawback.

Daemonic Influence is more of a trap - it cuts off the versatility of the card and is an obscene amount of XP that needs to be spent on other keystone 5 XP friends.

On other characters, this one's tough. The arcane or ally slot needs to be dealt with to really maximize usage, and Daemonic Influence is probably the breakeven point, meaning that you're spending a full 10 XP to fully kit a monster to eat your Renfield or Acolyte and give you limited extra actions at what will likely be less than your normal values. Might be a decent pick for doom oriented investigators who need to be able to toss assets, particularly the Mystic who comes with it, Amina Zidane.

Of course, once you have one, there's a ton of things about this card that are unique, and I think its position as an immobile action generator is most interesting:

1. it can be left at hunter occupied locations to evade them endlessly, trivializing some enemies and even some scenarios. Ignoring Alert means its particularly good against Vengeance enemies and a certain forgotten age Snake.
2. It attacks aloof monsters, and you can add vicious blow to the skill check. It also occupies space, which helps with placement of Acolytes and empty location enemies (although I think you'd prefer to drop Acolyte ON the space)
3. It can live in hard to reach areas with bad EOT effects, as well as areas that constantly repopulate clues, where it will just get you a clue every turn. Scarlet Keys has a few of these.

Note that although Guardian of the Crystallizer enters play exhausted, it still enters play engaged with the investigator who drew it.

When an enemy card is drawn by an investigator, that investigator must spawn it following any spawn direction the card bears (see "Spawn" on page 19). If the encountered enemy has no spawn direction, the enemy spawns engaged with the investigator encountering the card and is placed in that investigator's threat area.

This is probably the hardest of the cycle to make work. However, there's one new tome in TSK, grim memoir, that works pretty well with this as it's both expensive and requires multiple skill tests. Using in a Daisy deck that also managed to find another good tome as a story asset.