How do you summon him?

You don't.

When does he come out?

He comes along during a boss battle later in the game. All the other times its just random isnt it? I could never figure out a pattern, he just came along sometimes, did his thing and buggerd off.

The amount of Dead Spirits you have in your Inventory raises the rate of which he'll appear, but apart from that, it's random.

I never knew that before. Interesting.

yeah it true bout the spirts increase his appearance

He is also linked to your luck stat the higher the luck the more often you shall see odin.

I never knew about the dead spirits though I gotta say

He is also linked to your luck stat the higher the luck the more often you shall see odin.

I never knew about the dead spirits though I gotta say

yup, luck is also linked.

and i never knew the thing about dead spirits.

I didn't know that luck or dead spirits affected it. That's cool. I may play again just to test that out! Haha!