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Cannot be Normal Summoned/Set. Must be Special Summoned by its own
effect, and cannot be Special Summoned by other ways. When a face-up
monster you control is destroyed by a card effect and sent to the
Graveyard (except during the Damage Step): You can Special Summon this
card from your hand. Other monsters you control cannot declare an
attack. Once per turn: You can target 1 Synchro Monster your opponent
controls; equip that target to this card. This card gains ATK equal to
the combined ATK of the monsters equipped to it by this effect. Once
per turn, during either player's turn, when your opponent activates a
Spell Card: You can negate the activation, and if you do, destroy it.
All "True Draco" and "True King" monsters on the field gain 300
ATK/DEF. The first time each Tribute Summoned "True Draco" or "True
King" monster would be destroyed by battle each turn, it is not
destroyed. Once per turn: You can destroy 1 other card you control or
in your hand, and if you do, add 1 "True Draco" or "True King" card
from your Deck to your hand.
Player A has Dragonic Diagram is already on field. Then player B summons Meklord Emperor Wisel. Then player A wants to activate his Dragonic Diagram's effect. Can player B negate it with Meklord Emperor Wisel's effect?
No, you activated the effect of a Spell Card, not the card itself. All instances of activating a Spell Card is activating a Spell Card effect, but it is not the case that all instances of activating a Spell Card effect is activating a Spell Card.