I didn't see a thread dedicated to new-new Limbus so here's one with some findings. I’ve continued to edit this post with new discoveries made throughout the thread.
For entrance and zone info, really the SE post is best:
https://forum.square-enix.com/ffxi/threads/62862-June-10-2025-%28JST%29-Version-Update?p=667175&viewfull=1#post667175
Obtaining the Key Items / Units
Kill mobs to fill the bar in the upper left. Seems to take 6-15+ mobs per floor, this increases based on the chosen mob level and number of people in your party. You continue to get units as long as you want to sit there and keep killing and refill the bar, but at half the rate. Whether this is comparable to the units gained by continuing to move floors and finishing the event to get the chest bonus is up a function of your personal kill speed and group size.
Each Temporary KI looks like "Apollyon SE #1 data", and you get 1 per floor. You keep them until you open a chest and then they are all lost. You get a KI for filling up the bar at the top of the screen on each floor. I didn't count but I think it requires like 10 mobs per floor (this apparently varies by level choice). Once you get a KI, you should get to keep it even if you leave Limbus. You would lose any progress on any un-completed floors, but this is like 6-10 kills so no biggie.
You can enter any section / floor at any time. You don't need to go in any type of order or "climb" if you're missing one of the upper KIs from a run that got interrupted.
Once you choose a level (119-130), I think you need to keep that level choice until you open a chest. If you adjust it after getting some KIs, you might lose them. The NPC dialogue isn't too clear on this.
There were no direct drops from mobs, of any kind.
Mob Details
All the mobs seem to have pretty serious DT, probably 50% (confirm by Brixy as 50%, per Air Knife expected enspell dmg). But they aren't hard. They hit like 130s. AoE/Cleaving is extremely nerfed.
Pulling mobs is weird. Any initial spell cast before initial aggro / dynamic level adjustment may land, but will not remain on the mob. You will not see a message to reflect this.
If you pull with silence (while it's un-aggroed), it appears the silence instantly wears without any message, likely due to the dynamic-level adjustment which happens as soon as you aggro (credit to Moonlightagb for this theory, I think it's correct). Silencing twice is probably needed. This doesn't matter much in a party, but if you're solo and mob density is high this effect could be pretty annoying. Other examples later verified by others: If you pull with Immanence, the skill chain will not complete. If you pull with Dia, you can sleep right after.
Mob respawn is on the order of a few minutes but I was pretty lonely this morning. I had one floor with a couple groups and it felt faster, might be dynamic spawn rate.
Aside from the initial-aggro weirdness, everything can be debuffed and slept as expected. Sneak/invis work fine, but true aggro mobs remain like Imps and such.
Getting the goods
In order for the chest to open,
you need to finish all of the zone, not just one section. So Apollyon zone is 18 KIs, Temenos zone should be 25 KIs. If the chest says "it won't open", you need more KIs. The chests are located at the exit of each "top floor" in each section. However, you can only open a chest once you have finished all the sections (NW,NE,SW,SE,etc).
After getting all KIs in Apollyon, each character had earned:
About 4000 units from killing mobs and climbing floors
3000 units from finding a shiny ??? on the ground and clicking it
3000 units (one character in the party got 5000 units, no idea why) directly from the chest
So each character of a 6-person party left a ~90 minute run with 10-12k units. EP was ~15k/hr, but this is ML50s fighting level 130s. Maybe it's better if you're lower.
My initial limit on Apollyon units was 30,000. After I opened the chest, that increased to 33,000. It looks to increase 3000 more with each chest open.
Apollyon Units are used to upgrade AF pieces to +4.
Temenos Units are used to upgrade relic pieces to +4.
The upgrade costs are:
Head: 20k units
Body: 30k units
Hands: 15k units
Legs: 25k units
Feet: 10k units
Once I got to 10k units, the furnace did allow me to trade a foot piece, though I didn't choose to complete the process. Trading any other slot did nothing.
I did get the title Apollyon Razer, which I think is just the same as the old title lol
