Devs spawned bosses at Margle Palace on Greybriar (NA) shard as at around 5:30pm PST on 23rd Dec 2014.
Started off with a cluster-wide server announcement on all North American shards:
Strange planar energies begin to gather at Margle Palace on Greybriar!
The lag/fps drop was intense with the massive amount of players that came to Margle Palace on Greybriar.
Laethys in Margle Palace Fae Yule
First, Laethys was spawned but reached about 72% hp before it bugged out. The devs then summoned The Golden Devourer. It seemed to have died although players were claiming they didn’t get the quest completion for it. Apparently dragon pinatas were spawned although there were so many people it was hard to tell. Some players received Level 60 loot in their Rift Loot Bag (I got Empyreal Slayer’s Marks and a Level 60 essence).
The Golden Devourer in Margle Palace Fae Yule
DEV_Pithos has since ended the event but stated that Trion were able to get some good performance information from it.
Hopefully this helps towards improving performance in the future. Perhaps this was to test out how Rift would perform for an opening event (or zone events) for the upcoming island (Tyrant’s Throne) with Patch 3.1: Storm at Sea?
[Various bosses also appeared later that day on Wolfsbane, Laethys, Faeblight and likely other shards as well.]
RiftGrate 
I didn’t get anything but massive lag, but it was still a bit of fun and a break in the norm. Hopefully, they did get some good info.
Yea the lag was intense. I think for the Golden Devourer I only got off a few hits and I was never in combat.
It’s a shame that there were so many players but I only see around 20 or so on my screen at any one time.
I’m hoping Tyrant’s Throne has some really nice zone events like Ember Isle had.
That would be great!
and too bad that all those player who participated to this got NOTHING what was that? all the efforts they put into is just wasted :3 atleast give something for all players random 10players is lame do something about it atleast faelight or marks
Aww crud, I missed it. I mistakenly thought it was supposed to happen on the 27th
The Golden Devourer player-held kill event is happening on the 27th but the devs spawned him randomly in Margle. The version in Margle didn’t give any rewards so you didn’t miss out on much.