It looks just plain OP to me at the moment.
Here are the only cards that are able to break through the self-Cloaking:
Thunderstorm (rarely used)Plague (rarely used)Retrovirus (rarely used)Rain of Fire (costs more quanta; will likely come out later than the Assassin will)Unstable Gas (still rather expensive to use)Pandemonium (the only real sensible counter)
Letting the bonuses stack when Cloak is played is just going way over the top.
It would be more balanced to let it start uncloaked and give it a small (read: maybe +2/+0) bonus on when it does get Cloaked.
In the notes I said that ANY effect will get the cloak of. Delay, freeze, fire dmg from buckler, poison from shields, ETC. Besides what do you mean with letting the bonusses stack? He only gets +4 dmg once even if you play cloak while he's cloaked.
Plague is not rarely used! I played 5x pvp2 and got 3x rainbows with plague. Bad luck?
5 for an 8|4 creature that can't be hit without an AoE? Definitely not worth it. A cost of 7 or 8 suffices.
So you would actually pay 7/8
for a 8|4 creature that becomes 4|4 as soon as almost any shield/CC is put up?
I did think of getting a +3|0 bonus as +4 is maybe a little OP but still there are a LOT of counters against this. It hasn't momentum and if you give it momentum the cloak goes away and you have to put up your own cloak, so dim shields/wings counter it good.
about the OP: shrieker is 3
and 1
and has permanently 10 dmg. 10 dmg for 4 quanta! Yes he's visible but he will keep his dmg no matter what. Both units have 4 hp. (+shrieker can burrow which gives him 5 att and immortality).