It's not really clear if he ever was a child. He'd been weaker, certainly, and... probably smaller. Mostly though, he was made to serve.

The first he serves is his creator, the Shadow Priestess Leandra. She is detached and curious, but doesn't really have any particular expectations of him. Mostly she just... wants to see what he can do. Judge what he is, if her experiment was successful. How strong is he, how smart is he, how quickly does he reconstitute after total destruction, how far can he get from the heart that binds his soul to this world, that sort of thing. She's very methodical. The tests are sometimes uncomfortable or unpleasant, but they're genuinely very informative towards his abilities. Invisibility and intangibility come naturally to him, and while he is bound to his heart and whomever holds it, he's only constrained to being near it when he's been ordered to. Even if he's intangible, silver will connect with him as if he were not, but the priestess nonetheless seems to be very pleased by this result. She's unsurprised by his weakness to sunlight, and finds the lethargy and weakness it inflicts upon him academically interesting, but nothing else.

He forges his first weapon for her, too; a small, unassuming dagger that she chanted and sprinkled some sort of glowing powder over while he hammered hot metal into shape. This is because she's unable to do such physical labor herself; Leandra is an old and withered thing that can barely lift her own staff to cast the magic she's so skilled in, let alone accomplish the forging of a blade. And the obvious thing to give a servant that can turn invisible and intangible is a blade to match, so a blade he gets. It can, with a bit of finesse from his creator, turn invisible and intangible with him. She doesn't explain the process, but he's there when she experiments and iterates upon it, making the perfect deadly weapon for a phantom. It is a straightforward enough thing for him to grasp.

Then, with a weapon that is as suited for spying and assassination as he is, she puts him to use.

For a while, anyway.