Legion Warboss

Mana cost: {2}{R}
Type: Creature — Goblin Soldier
Power / toughness: 2 / 2
Set: Ravnica Remastered #337z (rare)

Mentor (Whenever this creature attacks, put a +1/+1 counter on target attacking creature with lesser power.)
At the beginning of combat on your turn, create a 1/1 red Goblin creature token. That token gains haste until end of turn and attacks this combat if able.

Legion Warboss has 8 printings, first in Guilds of Ravnica (2018). EDHREC ranks it #1,794 among Commander cards. It appears most in Krenko, Mob Boss decks (19,680 on EDHREC). 2 public decks on TCG run it. It has 4 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run Legion Warboss

Mardu Surge - Tarkir: Dragonstorm Commander, led by Zurgo Stormrender
Rebellion Rising - Phyrexia: All Will Be One Commander, led by Neyali, Suns' Vanguard

Commanders that run Legion Warboss most

Deck counts from EDHREC.

Krenko, Mob Boss: 19,680 decks
Winota, Joiner of Forces: 9,863 decks
Zurgo Stormrender: 8,092 decks
Zada, Hedron Grinder: 4,387 decks
Ingris Stingerquill: 2,283 decks
The Great Goblin: 2,029 decks
General Kreat, the Boltbringer: 2,001 decks
Grub, Storied Matriarch // Grub, Notorious Auntie: 1,809 decks

Often played with Legion Warboss

Cards that show up in the same decks on EDHREC.

Rulings

2018-10-05: If the creature with mentor leaves the battlefield with mentor on the stack, use its power as that creature last existed on the battlefield to determine whether the target creature has less power.
2018-10-05: If the target creature's power is no longer less than the attacking creature's power as the ability resolves, mentor doesn't add a +1/+1 counter. For example, if two 3/3 creatures with mentor attack and both mentor triggers target the same 2/2 creature, the first to resolve puts a +1/+1 counter on it and the second does nothing.
2018-10-05: If the token can't attack for any reason (such as becoming tapped), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.
2018-10-05: Mentor compares the power of the creature with mentor with that of the target creature at two different times: once as the triggered ability is put onto the stack, and once as the triggered ability resolves. If you wish to raise a creature's power so its mentor ability can target a bigger creature, the last chance you have to do so is during the beginning of combat step.