This is a personal, largely spoiler free (or spoiler-vague) review of the fan-made scenario Event Horizon, part of the Mythos Deviations collection by The Beard for Arkham Horror the Card Game.
The Mythos Deviations are a collection of scenarios design to be added into the existing official campaigns. Event Horizon is designed to be played as part of the The Circle Undone, after The Secret Name (3rd scenario), The Wages of Sin (4th scenario), or For the Greater Good (5th scenario), or as a standalone scenario or side scenario as normal. Event Horizon has the investigators trying to stop a void consuming the French Hill district of Arkham.
The thing that characterizes Event Horizon more than anything else is that it's fast. The scenario gets started quickly, showing off its primary mechanic of disappearing locations starting on turn 3 - if not sooner, depending on your encounter cards. You're given little time to set up and get moving before you start losing opportunities, and each disappeared location ends up hurting a lot. The threat of Daemonic Piping breathing down your neck also gives a good motive to just keep moving.
For intergrating into The Circle Undone, it's a mixed bag. There are a lot of uses of the momentos in the scenario, which is a welcome addition. Some of them, however, are on fairly soft enemies - the nastiest monster in the encounter deck is the classic Hunting Nightgaunt! The scenario focuses more on Azathoth-themed effects than witches, ghosts, or silver twilight, so it ends up not really feeling like any of the existing Circle Undone scenarios thematically - but it does lead into the last two scenarios and the finale better, which always came out of nowhere. It's a fun scenario on its own, though, with a lot of considerations as to how you're going to tackle it - as long as you do so quickly.
With a 19 XP party, I found Event Horizon to be about right. Being able to have everyone quickly get clues will certainly help, as an early lead in the scenario can pay off big-time. The monsters are mostly soft enough (save the boss, and the Piper of Azathoth if you're unfortunate enough to draw it) that you can get by with lighter fighting.
While it doesn't quite feel like Circle Undone mechanically, the scenario's got a strong backbone and a good hook - and might be a welcome break in between some of Circle Undone's more complicated scenarios.
Rewards
Scenario Cost: 2 XP, or 1 XP as part of a The Circle Undone campaign
Potential XP Gain: 2 (locations) +2 (Piper of Azathoth, summoned by Daemonic Piping in the encounter deck) +1 (Singularity Harbinger) = 5 XP or 3 XP after cost, or 4 XP if part of a The Cicle Undone campaign
In addition, on scenario success one investigator may choose to add Void Mantle to their deck.
More on Mythos Deviations:
Mythos Deviations: Overall
Lines of Reality: Review and Rewards
Pictures at an Exhibition: Review and Rewards
History in the Making: Review and Rewards
Event Horizon: Review and Rewards
The Labours of Hypnos: Review and Rewards
The Wreck of the Albatross: Review and Rewards
Mythos Deviations Investigators