IMO 'Arcane Slot builds' are still kind of a meme, though Dragon Pole and Explosive Ward are making them workable sub-themes, albeit in opposite directions.

That said, the biggest perk here is taking enemies out of play, especially in 4 Player games, where you burn through encounter cards at a fast clip. You'll run out of health or sanity, or bullets in your gun, and when faced with Alert or Retaliate keywords, the worst enemies become huge action sinks over time, outpacing many treacheries. Some treacheries exist specifically to throw them right back at you. The Jar lets you shove the most annoying of them in something like Diana's basement, but for monsters. It's the hardest "screw right off" to the horrors since Bury Them Deep. Eat lonely Deep Ones, spammy Snakes, and dancers in the void. The biggest downside is having to be present for the monster's original death.

This card seems really good, the only drawback is that it takes the coveted mystic accessory slot.

First up: It's dead cheap. Cost 2 is very reasonable for the resource starved class and for the effects it's a bargain. 2xp for 2 copies helps as well, even if it's Unique. Since it's level 1 you can even take it in off-class Mystics, which might be very interesting in something like a Guardian who isn't using the accessory slot as much and will have no problem defeating enemies.

The first effect is why you are buying this card, getting rid of Acolytes, Wizards of the Order and the like out of the discard pile for the scenario and getting yourself more Arcane slots for the privilege is great. Is it worth using over Holy Rosary or St. Hubert's Key? Hard to say, I'm keen to play around with this one though.

If you need more arcane slots in a resource hungry build, the economy of this card is game changing. For just 1 resource and half an XP per slot, you can necro 2 extra arcane slots out of whatever monsters you can stuff into this interdimensional jar. It's as if those "pickle me" Elmos they sell at the flea market filled you with eldritch power, and occasionally let you stop an attack in its tracks as a failsafe.

I don't know exactly what you need all those cheap arcane spaces for, but I bet you know a mystic who wants them!

For comparison, here are your other options for getting another arcane slot:

• Sign Magick Costs 3 resources. New slot is limited to holding Spells and Rituals. FAST.
• Familiar Spirit Ally. Only costs 1, but takes up the ally slot and offers minimal other benefits. New slot is limited to holding Spells.
• Dragon Pole. Weapon. Costs 3, and takes up both hands slots.
• Book of Shadows Costs 3xp and 4 resources. Takes a hand slot. Adds a charge to spells.
• THE HIEROPHANT · V Costs 3 resources and 3xp per slot. Also lets you share slots between accessories and arcane. Takes the tarot slot.

Another item designed for Lily that she can't take. I'm going to say that Lily's deckbuilding parameters are a complete mistake and make her unfun to build. She's the only investigator we've just house ruled to be Guardian 0-5, Mystic 0-2. She's just a waste of potential otherwise.