Cold-Eyed Selkie [Commander 2015] 026 It has "Sacrifice this creature:

It has "Sacrifice this creature: Add C

where X is the sacrificed permanent's mana value

Set: Kaldheim Type: Legendary Creature — Human Warrior Rarity: Uncommon Cost: {2}{G}{W}{W} Other creatures you control get +1/+1

then I'd say we have an appropriately sensible populace

Set: Sword & Shield: Evolving Skies Type: Dragon Rarity: Ultra Rare Retreat cost: 0

Cold-Eyed Selkie [Commander 2015] 026 It has "Sacrifice this creature:Set: Commander 2015 Type: Creature Merfolk Rogue Rarity: Rare Cost: {1}{G U}{G U} Islandwalk (This creature can't be blocked as long as defending player controls an Island.) Whenever Cold Eyed Selkie deals combat damage to a player, you may draw that many cards. She follows the old tributaries built when merrows still shaped the tides.