Unlike some of the other romanceable characters in Hades, by indie developer Supergiant Games, Thanatos will not perpetually stay within the House of Hades for convenient conversation. Players will have to encounter him upon the battlefield for comradery and competition.
In any given run, at the beginning of every encounter, there is a small chance that Thanatos will spawn and challenge Zagreus to see who can slay the most foes. This will initiate a sequence where multiple waves of enemies spawn one after the other while both the player and Thanatos each try to take down as many as possible to prove which of them is the most powerful.
After each run where Thanatos is encountered, he will most likely appear within the House of Hades, though, it is not guaranteed, as the personification of death is quite busy and may not always have time to chat. When Zagreus can talk to him, however, players can deepen the bond between the two mighty warriors in Hades with specific gifts.
Step 1 – Give Thanatos Nectar
Giving Nectar & Getting The Pierced Butterfly
Players can give Thanatos Nectar when encountering him in a run or afterward if he can be found in the House of Hades, however, keep in mind that if he is given some Nectar in a run, he won't accept any the next time he appears in the House of Hades, as, to the game's engine, these somewhat count as a single encounter.
The first time that the player gives Thanatos some Nectar, he will gift Zagreus the Pierced Butterfly, which is a Keepsake that will increase the amount of damage the player does each time that they clear an encounter without taking damage.
One will be able to give Thanatos 6 Nectar before he will not accept any more. After this point, players will need to perform a Favor for him before being able to deepen their bond any further, however, this task is not really challenging to find or perform as it is basically just a dialogue event that will occur naturally.
After giving Thanatos 6 Nectar, the next time the player encounters him in a run, he will speak some dialogue about the player being strong enough to get out of the Underworld by themselves. After this encounter, the next time he is found in the House of Hades, players can begin giving him Ambrosia.
Step 2 – Give Thanatos Ambrosia
Giving Ambrosia & Getting Companion Mort
Similar to giving Nectar, players can give Thanatos Ambrosia whenever they spot him in a run or afterward in the House of Hades. The first time the player gives Thanatos some Ambrosia, he will give Zagreus Companion Mort, who can be used in battle to summon Thanatos for a brief yet powerful strike.
Fully Romancing Thanatos
It will take 4 Ambrosia after performing Thanatos' Favor to reach maximum Affinity with him. Afterward, the personification of death can be found in Zagreus' room, where the player will have to make a decision between staying friends with their fellow warrior or becoming lovers.