I had intended this to function more like James described in his reply, where you have to suffer through an opponent's creature growing in order to obtain the benefit of gaining control of it. The card would be more powerful with a form of protection, but that would defeat the purpose that I had in mind creating it. You are right about it being slow, though. I'm going to change the P/T from 3/4 to 2/6 to to make Sevla a better blocker, which will hopefully help defend the player from creatures with pact counters. This will hopefully make the card better at outlasting during the slow game plan it demands.
In all likelihood, it wasn't designed that way but I just thought it would be an interesting twist on the concept. The point of you stealing the creature would be so that you can use it yourself, especially if it has an activated ability, rather than the creature's owner deciding whether and when to use it. Also, you controlling the creature means your opponent doesn't have it anymore at their disposal, for offense or defense.
Given how incredibly slow the card works (you don't get to steal anything without proliferate assistance until three turns after you cast it), having something work in your favour during the intervening turns would make the card more playable instead of repeatedly enhancing their best creature and you having to live with it unmitigated that whole time. A stepwise progression also makes sense flavourwise: as the creature gets deeper into conforming to the pact, it goes from being against you to nonaggression (like putting Assault Suits on your opponent's creatures or having a Combat Calligrapher on your side) to being for you.
I'm not sure if that is how the card was to be designed though. You're supposed to experience the downside of the card in order obtain the effect of stealing the opponent's creatures. Protection or goad would be interesting, but why steal the creatures if you can just permanently goad one after the other? It would ruin the third abilty.
That's interesting because I was thinking that Sevla should have protection from creatures with pact counters on them or give you protection from them, kind of like how goad plays people off each other while you sit back and watch. But that necessitates a much more expensive card.
Nice design! I like how prolfierate can work this mechanic very nicely! I would suggest maybe rewording the second abilty to "Creatures get+1/+1 for each pact counter on them" so when you gain control of a creature, you would still get the benefit to keep proliferating the pact counters to keep buffing the creatures up. Maybe you did this to keep the card balanced, but I think this way it'll add more layers to the card, even if you need to increase it's mana cost.