After reading several topics on a subject wasn't able to resist the urge to test it out myself.
Spoilers ahead.
You're starting out with collecting vials from overland encounters with Seekers, if I remember correctly they're 1.8 mil health total so roughly an equivalent of a base game World Boss, difficulty wise it reminded me a Crowborne Horror boss from a recent Witches festival group quest addition, but with much less health to it and just one (time dependent?) mob phase. After you've collected all five, quest directs you to the Dolmen+ marker and you're ported to the Bastion itself using previously mentioned liquids.
Next phase heavily reminded me of Deadlands portals, but finally it's instanced so no more running past piles of corpses or wasting your time for a respawn. Difficulty wise it's probably just the same, might check hp values later. Met only one above trash tier mini-boss on my way and it's way less of a problem than Deadlands are having between zones with no potential to really harm you, it had around 800k health and melted without an issue along other mobs, as i noticed him in the very last moment.
Not sure if there's more different versions of it (hope so?), but what i've got were two boss encounters.
First one being a Lurcher with a pair of Watchers all around 2mil health (Lurcher being main encounter with Crowborne health pool of 2.2mil and supports of 1.8mil). Quite enjoyable fight and reminds a lot of a solo arena than a dungeon or other encounters, but more prolonged one due to their health pools.
Second one was a Seeker yet again, at a whole 5 mil health so an equivalent of a DLC World Boss, if I remember correctly, and it was a fun one. One thing that sets it apart from a WB is it's low damage output and slow reaction times.
Still a hard feeling of a solo arena so it was either a scrap of the Dungeon DLC that we'll never get this year or a whole Solo Arena that was merged with Incursion event for some reason.
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Overall it was way more enjoyable than a Havocrel encounter in Deadlands or wandering bosses of Galen, those weren't even remotely interesting alrerady after half of the first encounter with them. And it's an improvement after a public dungeon approach of Oblivion Portals, which led to camping and a lot of waiting around for mobs\Duke. Aesthetically it was close to flawless, which isn't the case a lot of times with such content but probably not exclusive to this event, but to Apocrypha depiction overall which I didn't visited. Also no irritating NPC callouts or taunting of a Big Baddy Behind the Scene was a breath of fresh air, especially after HI fissures.
The only downside I can possibly think of is inclusion of in city daily quest and an Incursion icon to it. It's misleading as people generally expect some bite size open world event to melt with random people around and not a solo arena instance at the end of it. Which I personally enjoyed a lot, but it might get people confused. Even an arena\trial\dungeon icon would do lot better describing what people should expect of it, but this might matter less with the next update after Chapter where the new variety group finder will be in place. The lack of usual "dolmen" event is a good thing for me as those always were a sore of a zone, so to say, deforming the beauty of zones and making for another place people are driving past through. This format is way more fitting but forced grouping for new players might be a double edged sword after the content would become less and less fresh. I'd suggest no daily for it at all, making for a more adventurous approach not being directed to the task but finding it yourself at your own pace.
If people are worry about the difficulty, i'd personally rank it lower than a Harrowstorm, as you're getting bosses piece by piece and have a lot of breath room, but quite above Summerset geysers.
Don't forget a self heal this time, it's ain't a dolmen this time around.