Amumu is very OP after his buffs last patch, so Riot is already looking to nerf him
Amumu was one of a few weak junglers targeted in League of Legends' last patch for major buffs. He definitely needed the help, but what he got was a little too over the top. Luckily, however, Riot is looking to nerf him pretty hard in the next patch.
When we first saw the buff on the PBE before the patch went live, we were a little skeptical it would last very long due to how powerful it was. The buff would rework Amumu's passive to cause enemies he hits with basic attacks to take 15 percent of all incoming magic damage as true damage instead. If you think about that for just a moment, it isn't hard to see why that's OP.
Lux ultimates, Syndra ultimates, Katarina ultimates, and all other magic-based spells that already shred through champions like butter could now deal true damage if they had a certain mummy on their side. On the one hand, it was refreshing to see more mages in the game that's been dominated by AD assassins lately. On the other hand, watching a Syndra ultimate alone take you from full health to dead just because an Amumu jumped on your face gets old pretty quick.
It didn't take long for Riot to realize that it had unleashed a monster, so nerfs to Amumu's strong passive have already been pushed to the PBE, League's player-testing client. The nerf started off by lowering the true damage conversion from 15 percent to eight percent, but that was a little too harsh, so it was brought up to 10 percent instead. Still, a five-percent nerf from current passive in the live client is a big deal, especially when we're talking about hundreds and hundreds in magic damage coming from single spells, like a Syndra ult.
For example, if Syndra's ultimate deals 800 damage, Amumu's current passive would make 120 of that deal true damage and completely ignore the target's magic resist. After the nerf down to 10 percent, however, it would deal 80 true damage.
As you can see, that's a big chunk of true damage being lost, and good riddance. If this nerf makes it through to the next patch, it will likely be out in a couple of weeks, as traditionally patches go live about three weeks apart from one another.
Amumu will still be in a much better place from prior to the buff, but he shouldn't be ridiculously OP anymore.