Read Know No Fear, in which we are introduced to Oll Persson. Dunno if I like the idea that some hithero unknown super-immortal thing is going to be the one what helps the Emperor cut down Horus.
Liked it better when it was a Custodes terminator, that was a little more believable, or a regular human troop, which is more relateable.

It being some random immortal guy who isn't something the Emperor did, makes it kind of jarring.

Writing was good, though