Companion Antos is obtained by reaching a high level of trust with Achilles, and then gifting him Ambrosia.
Using Antos in battle will summon Achilles and Patroclus to aid you. They will appear briefly and deal 1500 damage to 2 foes one after the other.
Antos cannot be used outside of combat encounters or while fighting Hades. Hades will eventually grant the authority to use Antos against him.
Like other Companions, Antos 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 Antos first emerged from the darkness, he smashed through the very earth, heaving boulders many times his size until he stood on all six legs, victorious. He felt prepared to take on any challenge. However, Mother Night urged him to be mindful of his limits; there is always something bigger, something stronger in the world. Antos was undeterred. If he could sweep Night herself from her feet surely she would recognize his power, and apologize. He slid under the rocks and earth, lying in ambush until just the right moment. When Mother Night crossed his path, paying him no heed at all, he seized his opportunity to prove his surpassing strength. He struck Mother Night full in the ankle. But, she merely turned to him, and sadly shook her head. Now, Antos still possesses strength many times his size; though he is very small, indeed.'
Once Zagreus forges a bond with both Achilles and Patroclus, the text becomes "From Achilles & Patroclus, their devotion, absolute"
Antos' appearance is likely a reference to how Achilles was the commander of the warrior tribe known as Myrmidons, a word that has been interpreted to mean "ant-people," from the Ancient Greek μύρμηξ