This card is treated as a Normal Monster while face-up on the field or in the Graveyard. While this card is face-up on the field, you can Normal Summon it to have it become an Effect Monster with this effect.● Once, while this card is face-up on the field: You can place 1 Spell Counter on it. This card gains 300 ATK for each Spell Counter on it. You can remove 1 Spell Counter from this card, then target 1 monster on the field; destroy that target.
This card further grants your Dark Valkyria additional Spell Counters so you can use it to remove your opponent’s monster. You can use the Spell Counters on Magical Citadel of Endymion Field Spell, as a cost instead, to use Dark Valkyria’s effect.
Supervise is probably the best Equip card to be used with Dark Valkyria, because it can directly make this monster an Effect monster, and you can directly place Spell counters on it.
Birthright and Soul Resurrection are able to Special Summon any Gemini-type monster to the Field from the Graveyard, in ATK or DEF position respectively.
Swing of Memories allows you to Special Summon any Gemini-type monster to the Field from the Graveyard, but it is destroyed during the End Phase. Blazewing Butterfly’s effect allows it to be sent to the Graveyard to Special Summon your Gemini monsters to the Field, and they become Effect monsters.
Gemini Summoner can Normal Summon/Set any Gemini monster from your Hand to the Field by paying 500 LP, during your opponent’s End Phase.
Create a Gemini deck with Supervise, Gemini Spark, Gemini Trap Hole and other decent Gemini cards like Blazewing Butterfly and Evocator Chevalier. Haven't got a decklist but it would be easy enough to make.
I would probably run Balance as the skill and control trap cards like Canadia and Drowning Mirror Force.
Sounds like alot of effort to make it broken. 2 summons, a field spell, plus any spells u have to activate to use endymon. Just use a that orochi reptile thing instead, banish 1 reptile and u get to pop a card eqch turn as long as u keep it alive