how does this work

it doesnt

Unfortunately this. Was one of the first things I tried with Justicar because I wanted to make Shadow priest viable again.

If you Shadowform first it doesn't do anything at all. If you Shadowform after it just turns your 4 heal into 2 damage.

Prophet Velen though

Trueheart Gain control, play velen and 1 turn later auchenai that's the closest you get.

read the card.

replace a "starting" hero power with a better one.

shadowform hero power isn't a starting one.

this