Effect:
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 be treated as an Effect Monster with this effect:
● One time only, you can place 1 Spell Counter on this card. This card gains 300 ATK for each Spell Counter on it. You can remove 1 Spell Counter from this card to destroy 1 monster on the field.
Some tips and useful things to note:
This card and Crusader of Endymion both Gemini Summon then you can use this card effect and power both of them at the same time.
The main thing I love the most is that you can actually use Unleash Your Power! To have multiple use of her effect.
This card makes an amazing combo with Tempest Magician. Since Tempest Magician can place Spell Counters at the cost of discarding cards, you can place them on Dark Valkyria's effect to boost her attack or clear the field.