Rakdos, Lord of Riots

Mana cost: {B}{B}{R}{R}
Type: Legendary Creature — Demon
Power / toughness: 6 / 6
Set: Commander Legends #532 (mythic)

You can't cast Rakdos unless an opponent lost life this turn.
Flying, trample
Creature spells you cast cost {1} less to cast for each 1 life your opponents have lost this turn.

Rakdos, Lord of Riots has 8 printings, first in Return to Ravnica (2012). EDHREC ranks it #2,678 among Commander cards. It appears most in Be'lakor, the Dark Master decks (10,767 on EDHREC). 1 public deck on TCG runs it. It has 7 official rulings.

Legality

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

Public decks that run Rakdos, Lord of Riots

Endless Punishment - Duskmourn: House of Horror Commander, led by Valgavoth, Harrower of Souls

Commanders that run Rakdos, Lord of Riots most

Deck counts from EDHREC.

Be'lakor, the Dark Master: 10,767 decks
Valgavoth, Harrower of Souls: 10,336 decks
Kaalia of the Vast: 5,627 decks
Raphael, Fiendish Savior: 3,065 decks
Abaddon the Despoiler: 2,869 decks
The Lord of Pain: 2,452 decks
Gev, Scaled Scorch: 1,307 decks
Ares, God of War: 1,123 decks

Often played with Rakdos, Lord of Riots

Cards that show up in the same decks on EDHREC.

Rulings

2020-11-10: Rakdos's last ability cares about the total life lost, not necessarily what an opponent's life total is compared to what it was at the beginning of the turn. For example, if an opponent loses 5 life and then gains 10 life in a turn, creature spells you cast will cost {5} less to cast.
2020-11-10: Damage dealt to a player causes that player to lose that much life.
2020-11-10: Rakdos can be put onto the battlefield by another spell or ability even if no opponent has lost life that turn.
2020-11-10: Rakdos's last ability won't reduce the cost to cast Rakdos itself. It applies only to creature spells you cast once Rakdos is on the battlefield.
2020-11-10: If there are additional costs to cast a creature spell, such as a kicker cost or the commander tax, apply those increases before applying cost reductions.
2020-11-10: If an opponent lost life and subsequently lost the game, Rakdos's first ability is satisfied, and its last ability counts that loss of life.
2020-11-10: Rakdos's last ability can't reduce the colored mana requirement of a creature spell you cast.