How to Summon Thanatos
Gather all 4 Fragments:
Fragment of Agony (Dropped by Thanatos Dolor)
Fragment of Despair (Dropped by Thanatos Despero)
Fragment of Hatred (Dropped by Thanatos Odium)
Fragment of Sorrow (Dropped by Thanatoes Maero)
Once you gather all 4 fragments you will have to go to thana_boss and place the 4 Fragments in the correct slot.
After you place all 4 Fragments in the correct spots the 4 monsters you killed to get those Fragments will be summoned and you must kill them.
After that Thanatos will be summoned in the middle (thana_boss 139 129) and after that you can kill him.
Guide courtesy of [GM]Seraphine