Ethereal Soul-Trader is a TCG card. You might find it in Auction House for 40-300k gold.
When you kill an enemy, real player or mob, the pet will create a purple lightning that will flow from the enemy. That is basically the reason why you should get this pet.
It works in battlegrounds and world pvp.
For real players, if they hit the release button (after they died), the thunder effect will get canceled.
Against normal mobs, the thunder effect lasts for around 5-6 seconds, more than enough.
Another problem is that if you play a class that has stealth, keeping the pet up all the time would be a bad idea, mostly for rogues. For warriors, paladins etc, there should be no problems.
And another problem. Summoning a pet triggers a global cooldown. You can’t avoid the global cooldown, even if you macro the summoning command with another ability.
Summon pet macro:
/summonpet Ethereal Soul-Trader
A macro for dismissing your pet:
/dismisspet