Project PH03-NIX is a legendary construct unit that has the ability to respawn itself as long as it dies away from your or the enemy's base. The starting strength and the strength it respawns at can be increased when leveled up. Project PH03-NIX fits well into construct decks.
Project PH03-NIX can keep bringing itself back and giving yourself value for a very long time until your opponent can destroy him at a base.
In order for your opponent to completely destroy Project PH03-NIX, they need to either wait until it reaches their base, or destroy him after it respawns again, which can put lots of pressure on your opponent.
Since Project PH03-NIX respawns in a random location at your base, you can place structures or units there to control its placement should you want to.