Companion Mort is obtained by reaching a high level of trust with Thanatos, and then gifting him Ambrosia.
Using Mort in battle will summon Thanatos to aid you. He will appear briefly and deal 3500 damage in a wide, circular area in front of you after a delay. The AoE damage is of the same style as the delayed AoE attack utilized in Thanatos' encounters.
Mort cannot be used outside of combat encounters, during a Thanatos encounter, or while fighting Hades. Hades will eventually grant the authority to use Mort against him.
Like other Companions, Mort can only be used once per combat encounter, and up to 5 times per run (upgradable with Ambrosia) or more if the Night Spindle is bought from the Well of Charon.
'...When Mort first emerged from the darkness, he was frightened to discover he was on the smaller side of the variety of creatures that surrounded him. The world was such a dangerous place! However, Mother Night told him to take heart, and not to make too much about his size. It was not to his detriment, to be so small. Mort decided to put the idea to the test. He discovered there were creatures even smaller than he; insects, arachnids, and the like. And so he lorded over them, using his size to his advantage. When Mother Nyx found him, he was huddled in a corner, surrounded by tiny creatures thoroughly terrified of him. Since then, Mort still retains his odd ability to instill fear, though as protector of whomever he is given to, not as tormentor of smallest things.'
Once Zagreus forges a bond with Thanatos, the text becomes "From Thanatos, closest colleague" or "From Thanatos, cherished partner"
It's implied in a conversation with Thanatos that Mort was a childhood toy of his. Zagreus recalls that Thanatos lost Mort long ago, but has seemingly found him since then.
The name Mort means 'death' in French, which is Thanatos' role.