When his mother sacrifices herself for his escape, a young boy must find magical body armour in order to defeat the vengeful spirits pursuing him.

Never start with "When," start with the protagonist. The conflict and the stakes are rather vague as presented. Yes it's very wordy, with lots of commas, which slow everything down, instead of being compelling, and there's no flow. A young boy must find his slain father's enchanted armour to preserve his humanity from a supreme being determined to rid him of it. Doesn't tell us everything but doesn't need to, just has to be interesting enough to encourage further reading. Does make clear the protagonist, antagonist, conflict, and stakes, which is all you need. Leave the extra details for a synopsis.

When his magical mother sacrifices herself for his escape, a young boy must find his slain father's enchanted armour to defeat his grandfather, a supreme being, who seeks to rid the boy of his humanity and sit alongside him in the heavens.

A bit wordy still but I think it's a little better.

The logline needs specifics to hang together better. Is the spirit angry he escaped. If yes the mothers sacrifice should be in the logline. If not dump her.

We need to see a connection between all the elements you are taking about.