Hands down: to me, this is the coolest standalone reward of the bunch.

One issue Arkham had, is that the most common story reward of standalones, or even the campaigns in general is Ally assets. This leads eventually to diminishing returns, as you have only so many slots you can put allies in and each card makes your deck bigger.

Granted, Midwinter Gala is big middle finger to the criticism that Ally story assets are an uninspired reward, given you can choose between 12 or so of them. But I think the real gem, pun intended, of the standalone is the jewel

Winning the scenario lets you keep the item, but instead of being part of yoru deck, it has the encounter deck back, and you can choose to shuffle it in every scenario you play, and once somebody draws it as an encounter, they get control of it and it generates 3 damage and 1 Doom.

The Doom parts is obviously the scary part, but it only matters if it would make advance the Agenda early, if you get it during the Witching Hour, this is basically a freebie, and that's before its ability comes in!

You can, as a free action, do a trivial Will or Agility test to move one token on it on an enemy at your location, be that damage (which would hurt the enemy) or Doom (which would give you a chance to remove it by defeating the enemy).

This is great, because not only the test has only a difficulty of 1, but also let's your choose between 2 different skill, and statistically, an Investigator will have an adequate amount of either stat you can test. And even if you fail, there is no drawback beside any effect outside of the jewel, like weaknesses or symbol tokens, it just fails to trigger. The Jewel gives you basically 1 free action damage in 3 different rounds: that is really damn good on its own, and can help finish off enemies with odd health or take out annoying aloof enemies like Whippoorwills without spending an action

Even the free action test is a benefit on its own, because it can be used as a "safe" test to trigger effects related to taking tests, like Drawing Thin or "Watch this!" or cash out on big rewards by passing a trivially easy skill check, like with Lucky Cigarette Case

If that was all the Jewel had going for, it would already been a fantastic high-risk/high-reward tool, but it also has the forced effect of getting shuffled back in the encounter deck when it leave play. That means the Jewel makes for a fantastic dummy target for discard assets effect like Crypt Chill, as that means you can recover it later, fully refreshed (albeit also with Doom as well, but given how easy is to work around the drawback, it's still much better to deal with that than most other encounters).

That's not even going over the great synergy it has with player cards: Doom Mystics can just Moonlight Ritual the Doom away to not worry about it, or even ready the jewel to use it twice with Sin-Eater. Seeker can just Eureka! at will on that. Survivor can similiarly use Resourceful to recover stuff. Guardians have arguably the least amount of direct synergy, but enjoy the main effect of the asset the most.

But by far the most broken combo comes with Rogues: anyone that can access upgraded Joey "The Rat" Vigil basically completely negates the drawback of the jewel, as he can just sell it away whenever the Doom would be a threat to anyone, or send it back to the encounter deck to refresh it with more damage tokens, while giving 2 Resources at pop. Joey can make it so the Jewel could replace not just one bad encounter, but multiple ones, per scenario. That is significant.

I love this thing: it's always a joy winning the scenario and bringing it to some campaign to spice things up, I honestly like it more than the standalone it comes from. The Jewel of Sarnath does bring some real risks, but overall the benefits far outweight the harm it may cause.