Riftbound Mighty
None of the 16 cards with Mighty print a reminder explaining it, so there is no official wording to quote here. Rather than guess at the rule, the cards themselves are below.
Which domains get it
Which sets print it
Every card with Mighty
Most expensive first. Prices are Cardmarket EUR.
Volibear - Relentless Storm
Origins
EUR 549.99
Fiora - Grand Duelist
Spiritforged
EUR 65.00
Volibear - Relentless Storm
Origins
EUR 54.00
Volibear - Relentless Storm (Origins Release Event Promo)
Origins
EUR 10.99
Kadregrin the Infernal
Origins · 9 energy
EUR 9.95
Fiora - Worthy
Spiritforged · 3 energy
EUR 6.75
Sacrifice
Unleashed · 1 energy
EUR 3.70
Fiora - Victorious
Origins · 4 energy
EUR 1.95
Sunken Temple
Spiritforged
EUR 0.21
Fiora - Worthy
Spiritforged · 3 energy
EUR 0.20
Volibear - Relentless Storm
Origins
EUR 0.14
Fiora - Grand Duelist
Spiritforged
EUR 0.09
Unsung Hero
Spiritforged · 2 energy
EUR 0.09
Jaull-Fish
Spiritforged · 7 energy
EUR 0.04
Show of Strength
Spiritforged · 2 energy
EUR 0.04
Unsung Hero (Spiritforged Nexus Night Promo)
Spiritforged · 2 energy
EUR 0.02