You can Special Summon this card from your hand by sending 1 face-up "Dragunity" monster you control to the Graveyard. When this card is Normal or Special Summoned from the hand, you can select 1 Dragon-Type "Dragunity" monster in your Graveyard, and equip it to this card.
You can send a Dragunity monster in your Spell/Trap Zone to the graveyard to summon Mystletainn. Dragunity Dux, Dragunity Legionnaire, etc. can equip a Dragunity monster to themselves which you can then use to summon Mystletainn.
Send a Dragunity Phalanx to the graveyard for Mystletainn’s summoning cost. Then equip Phalanx to Mystletainn and Special Summon the equipped Phalanx. This allows you to make a level-8 Synchro monster.