The requirement is
4 people 89+ party to enter Nidhoggur. Not necessarily transcendent, unless this has changed.
To get there:
Midgard Camp -> W -> N, eastern part of the map. There is a warper to save oneself.
--------
There are two ways to get there:
1) Do an access quest chain:
mandatory:
http://irowiki.org/oldwiki/Onward_to_the...orld_Quest
one of (or both, I suggest doing both as exp is really nice and quests are short and trivial):
http://irowiki.org/oldwiki/New_Surroundings
http://irowiki.org/oldwiki/Attitude_to_the_New_World
Basically, you need to be able to go New World W. This will happen at one point during doing one of the above quests (sorry, can't remember; too much time has passed since I made them).
2) Ask a nice, cute priest to save at Midgard 1W at the 'Kafra' (Cat Hand something) and get Warp Portal to that spot (you can get warp to any spot you are saved at even if the place is un-memoable). Go N from there and voila, you are on Nidhoggur map and can access the dungeon even if you haven't done the New World quest.
--------
What to expect (
spoilers inside):
Level 1:
If I remember correctly, Teleport is disabled there.
(note, not everything is listed below; just the information you might really want to know about if you enter that place first time ever)
Draco:
- Dispel (lots of Dispel)
- Fire Breath (not Kasa-level, but backlines should have Pasana/Jakk/Pneuma; sometimes you fight 2-3 Dracos simultaneously)
- Crimson Fire Formation (the flaming-ground-ninja-skill)
- Occasional Stun.
- Hits hard
Ancient Tree:
- Eswoo (yes, really)
- Casts Assumptio on friends, not on self
- Heals (9999) friends, not self
- Stuns
- Hits harder
Dark Pinguicula:
- Change Undead (frontlines need Bathory)
- Dispel (see a pattern? ;-) )
- Casts Status Recovery on frozen / stoned / stunned / whatever friends.
- Magic Mirror if MAtked sometimes (didn't notice a pattern yet)
- May spawn Nepenthes on death
- Ah, forgot. Hits hard.
Rata (miniboss):
- Hits very, very hard. Avoid tanking unless you
really know what you are doing.
- Basically, a combo champion squirrel
- Extreme flee
- Teleportable using a rudeattack
Use fire weapons in lv1.
If you pass level 1, I can explain level 2 if you want (but it is MUCH more difficult, so I'd suggest staying on lv1 for a while). In short: Change Undead comes from Dark Shadows, there are Rhynchos / Phyllas (water magic MAtk and, yes, Dispel) and Duneyrr (mister ouch).
---------------
Nidhoggur is basically a base-to-job converter. And my favourite dungeon, next to Gorge.
With discipline, me (at bolter prof - and trust me, I am NOT a good prof player), Aedra (before transcendence to High Priest) and T-chan managed to easily gain some exp and have fun there (avoiding Ratas though).
If people lack discipline... well, I've had 7-8 people parties there which managed to lose several % exp and not gain lots of job because people didn't listen ;-). A priest cannot save a reckless player there. Impossible.
The monsters - individually - aren't very difficult to kill, but they tend to swarm you (not as much as venatus though). With discipline you can avoid all deaths except of SURPRISE! attacks from behind when you are fighting something in front of you.
Nidhoggur1 is easier than it seems, but the party needs to be more careful. If anyone in party is reckless, this place will seem incredibly hard for the whole team.
---------------
If you want to have some more information about that place, fire me a PM. I'm away for this weekend but if you are willing to wait...