What the heck happened??

[Lestrade]

I was in tombs last night. Beginning of the match, almost all our teammates were poisoned??

We didn't do nothing much, but heck, we were pretty much poisoned from beginning to the end.. can anyone just tell me what happened??
Is there some necro spell that I failed to notice cause I didn't even see them casting anything...

[Feynt]

Cast Chilblains for no reason a few seconds before engaging the enemy, pick the one at the center of the enemy group and then use Plague Sending. A ranger with Poison Arrow can replace it where necessary on monks who cure their team mates, but that's basically the only thing I could think of.