Oona's Blackguard

Flying
Each other Rogue creature you control enters with an additional +1/+1 counter on it.
Whenever a creature you control with a +1/+1 counter on it deals combat damage to a player, that player discards a card.

Zendikar Rising Commander

Flying
Each other Rogue creature you control enters the battlefield with an additional +1/+1 counter on it.
Whenever a creature you control with a +1/+1 counter on it deals combat damage to a player, that player discards a card.

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If Oona's Blackguard enters at the same time as another Rogue (due to Living End, for example), that creature doesn't get a +1/+1 counter.

If a Rogue would normally enter with a certain number of +1/+1 counters on it, it enters with that many +1/+1 counters plus one on it instead. If a Rogue would normally enter with no +1/+1 counters on it, it enters with one +1/+1 counter on it instead.

The creature gets the counter if it would enter under your control. It doesn't matter who owns the creature or what zone it enters from (such as your opponent's graveyard, for example).

The effects from more than one of these cards are cumulative. That is, if you have two Oona's Blackguard on the battlefield, Rogue creatures you control enter with two additional +1/+1 counters on them.