Here's a very useful Final Fantasy XIV endgame guide by Setrien Nuen.
What you want to do at level 50 is unlock and begin running the level 50 dungeons, especially Amdapor Keep. While Castrum and Praetorium are faster Tombstone of Philosophy farms, Amdapor Keep armor is slightly better than your Artifact gear and while collecting it you are also farming Allagan Tombstones of Philosophy and Mythology.
Allagan Tombstones of Philosophy (ATP) tokens can be exchanged for Darklight gear and Allagan Tombstones of Mythology (ATM) for Artifact+1 from NPCs in Revenant's Toll - Mor Dhona.
Once you have your Darklight gear (or even while still working
towards a full set) you can start on your Relic weapon quest. This
requires you to have completed all of the main story line and done of
all the class/job quests for the specific weapon you are after.
1. Grand Company / Crafted - First level 50 weapon you want to get
2. Darklight - costs Allagan Tombstones of Philosophy
3. Primal - Ifirit < Titan < Garuda
4. Relic - requires a long quest chain and for you to beat all three HM Primals
5. Relic+1 - costs Allagan Tombstones of Mythology
6. Allagan - drops from Coil of Bahamut
1. Artifact (blue Class gear) / Grand Company
2. Amdapor Keep drops
3. Darklight - costs Allagan Tombstones of Philosphy
4. Artifact+1 - costs Allagan Tombstones of Mythology
5. Allagan - drops from Coil of Bahamut
What Happens at 50?
Once you reach level 50 you gain access to:
Castrum Meridianum - 100 Allagan Tombstones of Philosophy
The Praetorium - 100 Allagan Tombstones of Philosophy
The Wanderers Palace -
Amdapor Keep - 80 Allagan Tombstones of Philosophy and 40 Allagan Tombstone of Mythology
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