The Deep Dungeon - The Palace of the Dead!
As mentioned at PAX East this is a new type of content making its debut in FFXIV.
As you go deeper into the dungeon, the environment will look different. As you can see in the images, the weapon is glowing and that's the weapon you will be upgrading as you challenge this dungeon. There are items which can only be used inside this dungeon.
There are some items that can only be used in this content. Transforming monsters, all sorts of interesting stuff. Not something unique to the dungeon that you can take away, more things that only have effects in the dungeon. You'll also face off against bosses as well!
It can be played solo; however, it's balanced based on four players, so going in solo from the beginning will most likely get you killed if you're not prepared. There will be some challenge for going solo.
There will be duty finder matchmaking for this, too.
If you don't reach the next check point/auto save before being defeated, you'll return to the last one. The final glowing, special weapons can be taken out of the dungeon to be used in other content!
Also there will be some "special" bosses appearing on multiple floors!