Phoenix has a new kind of damage ability. It constantly burns all creeps in range. It starts out very weak, but you can buff Phoenix by using gold and spare Phoenix cards!
Thanks @Ghul for your idea!
Yeah, the added damage is flat and does not scale with level. This is different to Kiwi, it does not really deal attacks at all, but applies the DoT constantly.
I think he is talking about the yellow item not the set bonus. Which does not work, as Phoenix does not do attacks in the normal sense, the dmg increase does not proc.