1. "Magician's Valkyria" Activates: You activate the effect of "Magician's Valkyria," targeting all Spellcaster monsters on the field, including "Dark Magician," if it's present.
2. Spellcasters Flip Face-Up: All Spellcaster monsters on the field become face-up Attack Position, regardless of their previous positions. This includes the opponent's Spellcaster monsters, if any.
3. "Dark Magician" Gains ATK and DEF: "Dark Magician" gains 1000 ATK and DEF until the end of the turn. Its new stats become 2500 ATK/2100 DEF.
4. Taunt Your Spellcaster: You declare "Taunt Your Spellcaster" on your face-up Attack Position "Dark Magician." Your opponent can't target your "Dark Magician" with card effects or attacks.
5. "Magician's Valkyria" Restriction: Keep in mind that while your opponent can't target your "Dark Magician" directly, "Magician's Valkyria" itself cannot attack or activate its effects during the turn it uses its effect.
6. End of the Turn: At the end of the turn, "Dark Magician" returns to its original ATK and DEF, and its protection from "Taunt Your Spellcaster" ends.
Remember that the specifics of the interaction may differ depending on any additional effects or conditions in play, so it's always a good idea to carefully consider the current gamestate when applying these effects.