Grunn, the Lonely King

Kicker {3} (You may pay an additional {3} as you cast this spell.)
If Grunn, the Lonely King was kicked, it enters with five +1/+1 counters on it.
Whenever Grunn attacks alone, double its power and toughness until end of turn.

Dominaria

Kicker {3} (You may pay an additional {3} as you cast this spell.)
If Grunn, the Lonely King was kicked, it enters the battlefield with five +1/+1 counters on it.
Whenever Grunn attacks alone, double its power and toughness until end of turn.

Dominaria Promos

A creature attacks alone if it’s the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, Grunn’s last ability won’t trigger if you attack with multiple creatures and all but one of them are removed from combat.

If a creature’s power is less than 0 when it’s doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given Grunn -7/-0 so that it’s a -2/5 creature, doubling its power and toughness gives it -2/+5, and it’s a -4/10 until end of turn.

If an effect instructs you to “double” a creature’s power, that creature gets +X/+0, where X is its power as that effect begins to apply. The same is true for its toughness.