Thanatos Tower
Once you reach base level 80 you'll receive a quest to unlock Thanatos tower, a 12 man weekly instance (2 parties). You don't have to complete the questline to enter but the questline will ask you to clear floors within and will grant you some rewards in the process. You must be in a team (2 groups) to enter by inviting someone in a party to your party. Each party may only have one person if you like but a team must be formed in order to enter this instance.
The entrance to Thanatos Tower can be reached from Yuno at the Airship on the far right side of town. Speak to Karthaus and choose Go to the Thanatos Tower.
Once in the zone, head to the northern part of the zone and talk to Gageel to enter the instance.
To enter the instance you must be in a team. A team may be formed by creating a party, then inviting the leader of another party to form a team.
Overview
Thanatos Tower is a four floor raid with three difficulties:
Normal
Brave
Legend
To be clear, this is something you’re probably not going to solo on brave or higher for quite a while. This is difficult content which requires a few very geared people to clear this content if your group intends to carry people through it on brave or legend difficulty.