Confidence:StrongSolidPlausibleTentativeSpeculative

• Zaxara, the Exemplary

  1BGU

  Legendary Creature — Nightmare HydraBGU

  Both cards produce the same kind of effect (shared token subtype).
• Polukranos Reborn // Polukranos, Engine of Ruin

  GGG

  Legendary Creature — Hydra // Legendary Creature — Phyrexian HydraGW

  Both cards produce the same kind of effect (shared token subtype).
• Grakmaw, Skyclave Ravager

  1BG

  Legendary Creature — Hydra HorrorBG

  Both cards produce the same kind of effect (shared token subtype).
• Honored Hydra

  5G

  Creature — Snake HydraG

  Both cards produce the same kind of effect (shared token subtype).
• Llanowar Druid

  1G

  Creature — Elf DruidG

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Sprouting Phytohydra

  4G

  Creature — Plant HydraG

  Both cards produce the same kind of effect (shared token subtype).
• Innocuous Researcher

  3G

  Creature — Centaur DetectiveG

  Innocuous Researcher untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Fiery Gambit

  2R

  SorceryR

  Fiery Gambit untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Time Spiral

  4UU

  SorceryU

  Time Spiral untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Nissa, Vastwood Seer // Nissa, Sage Animist

  2G

  Legendary Creature — Elf Scout // Legendary Planeswalker — NissaG

  Nissa, Vastwood Seer untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Elrond, Moon-Reader

  2U

  Legendary Creature — Elf NobleU

  Hydra Broodmaster has activated abilities that aren't mana abilities, fueling Elrond, Moon-Reader's activation-trigger.
• Nissa, Worldwaker

  3GG

  Legendary Planeswalker — NissaG

  Nissa, Worldwaker untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Rewind

  2UU

  InstantU

  Rewind untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Prophet of Kruphix

  3GU

  Creature — Human WizardGU

  Prophet of Kruphix untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Bumi, Unleashed

  3RG

  Legendary Creature — Human Noble AllyGR

  Bumi, Unleashed untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Finale of Revelation

  XUU

  SorceryU

  Finale of Revelation untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Zacama, Primal Calamity

  6RGW

  Legendary Creature — Elder DinosaurGRW

  Zacama, Primal Calamity untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Sphinx of the Second Sun

  6UU

  Creature — SphinxU

  Sphinx of the Second Sun untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Wilderness Reclamation

  3G

  EnchantmentG

  Wilderness Reclamation untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Sword of Feast and Famine

  3

  Artifact — EquipmentC

  Sword of Feast and Famine untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Treachery

  3UU

  Enchantment — AuraU

  Treachery untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Beledros Witherbloom

  5BG

  Legendary Creature — Elder DragonBG

  Beledros Witherbloom untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Nature's Will

  2GG

  EnchantmentG

  Nature's Will untaps lands, fueling Hydra Broodmaster's mana-hungry effect.
• Rings of Brighthearth

  3

  ArtifactC

  Hydra Broodmaster has activated abilities that aren't mana abilities, fueling Rings of Brighthearth's activation-trigger.
• Illusionist's Bracers

  2

  Artifact — EquipmentC

  Hydra Broodmaster has activated abilities that aren't mana abilities, fueling Illusionist's Bracers's activation-trigger.