Rule 205.3j

Planeswalkers have their own unique set of subtypes; these subtypes are called planeswalker types. The planeswalker types are Ajani, Arlinn, Ashiok, Bolas, Chandra, Dack, Daretti, Domri, Dovin, Elspeth, Freyalise, Garruk, Gideon, Jace, Karn, Kaya, Kiora, Koth, Liliana, Nahiri, Narset, Nissa, Nixilis, Ral, Saheeli, Samut, Sarkhan, Sorin, Tamiyo, Teferi, Tezzeret, Tibalt, Ugin, Venser, Vraska, and Xenagos. If a player controls two or more planeswalkers that share a planeswalker type, that player chooses one of them, and the rest are put into their owners’ graveyards. This “planeswalker uniqueness rule” is a state-based action. See rule 704.

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