That's what I thought (the animation thing), but I will double check in client later.120 mana, which is a lot for him, and a very low HP regen. It's quite underwhelming even for passive while jungling/laning.
After checking the wiki I must admit I was wrong. The animation and sound are only visible to allies. Still that doesn't change the fact that you can easily play around it, you just need to think before acting.
It still is extremely difficult to play around. You can think before acting but you have no idea if he started Dark Pact or not. This turns all of that to a game of whack a mole on the highest difficulty.
Proven wrong above. That only plays for allies and that video is from Slark's team's perspective.