マルコフの刃の達人

If this creature deals combat damage to multiple players simultaneously, perhaps because some combat damage was redirected, its ability will trigger for each of those players.

The +1/+1 counter isn't put on Markov Blademaster in time to increase the amount of combat damage dealt during that combat step. However, the +1/+1 counter put on Markov Blademaster when first-strike damage is dealt will increase the amount of damage dealt during the regular combat damage step.