Confidence:StrongSolidPlausibleTentativeSpeculative

• Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight✓

  3WW

  Legendary Creature — Elemental God // Legendary Creature — Elemental GodBW

  Death replacement: a death-replacement grant on one card lets the other repeatedly die-and-return for combo (Mikaeus + Walking Ballista class).
• Esper Sojourners

  WUB

  Artifact Creature — Vedalken WizardBUW

  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.
• Najeela, the Blade-Blossom

  2R

  Legendary Creature — Human WarriorBGRUW

  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.
• Cacophodon

  3G

  Creature — DinosaurG

  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.
• Bloodline Recollector // Ancestral Craving

  1BB

  Creature — Vampire Warlock // InstantB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Bloodline Recollector's death-trigger or removal payoff.
• Chevill, Bane of Monsters

  BG

  Legendary Creature — Human RogueBG

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Chevill, Bane of Monsters's death-trigger or removal payoff.
• Private Research

  U

  Enchantment — AuraU

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Private Research's death-trigger or removal payoff.
• Quest for the Gravelord

  B

  EnchantmentB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Quest for the Gravelord's death-trigger or removal payoff.
• Bequeathal

  G

  Enchantment — AuraG

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Bequeathal's death-trigger or removal payoff.
• Kraven the Hunter

  1BG

  Legendary Creature — Human Warrior VillainBG

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Kraven the Hunter's death-trigger or removal payoff.
• Copperhorn Scout

  G

  Creature — Elf ScoutG

  Copperhorn Scout untaps all (or many) permanents, refreshing Clockwork Hydra's tap activation.
• Lagomos, Hand of Hatred

  1BR

  Legendary Creature — Human ShamanBR

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Lagomos, Hand of Hatred's death-trigger or removal payoff.
• Enduring Renewal

  2WW

  EnchantmentW

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Enduring Renewal's death-trigger or removal payoff.
• Isshin, Two Heavens as One

  RWB

  Legendary Creature — Human SamuraiBRW

  Isshin, Two Heavens as One makes Clockwork Hydra's triggered ability trigger an additional time — each time it fires, you get it twice. Value, not a loop.
• Eater of Virtue

  1

  Legendary Artifact — EquipmentC

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Eater of Virtue's death-trigger or removal payoff.
• Skullclamp

  1

  Artifact — EquipmentC

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Skullclamp's death-trigger or removal payoff.
• White Plume Adventurer

  2W

  Creature — Orc ClericW

  White Plume Adventurer untaps all (or many) permanents, refreshing Clockwork Hydra's tap activation.
• Malicious Affliction

  BB

  InstantB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Malicious Affliction's death-trigger or removal payoff.
• Athreos, God of Passage

  1WB

  Legendary Enchantment Creature — GodBW

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Athreos, God of Passage's death-trigger or removal payoff.
• Windcrag Siege

  1RW

  EnchantmentRW

  Windcrag Siege makes Clockwork Hydra's triggered ability trigger an additional time — each time it fires, you get it twice. Value, not a loop.
• Emeritus of Woe // Demonic Tutor

  3B1B

  Creature — Vampire Warlock // SorceryB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Emeritus of Woe's death-trigger or removal payoff.
• Ob Nixilis, Unshackled

  4BB

  Legendary Creature — DemonB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Ob Nixilis, Unshackled's death-trigger or removal payoff.
• Glissa, the Traitor

  BGG

  Legendary Creature — Phyrexian Zombie ElfBG

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Glissa, the Traitor's death-trigger or removal payoff.
• Incendiary

  R

  Enchantment — AuraR

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling Incendiary's death-trigger or removal payoff.
• The Meathook Massacre

  XBB

  Legendary EnchantmentB

  Clockwork Hydra can kill creatures (with damage or by shrinking them), enabling The Meathook Massacre's death-trigger or removal payoff.