When this card inflicts Battle Damage to your opponent by a direct attack: Draw 1 card. During the End Phase of the turn this card was Summoned: Send it to the Graveyard.
Rulings for Majestic Mech - SenkuYu-gi-oh! Card
If a "Majestic Mech" is face-down during the End Phase, its effect cannot activate so it will not be sent to the Graveyard. But it remembers when it was Summoned, even if flipped face-down. So if you Summon a "Majestic Mech - Senku", flip it face-down, then flip it face-up again during the same turn, it will be sent to the Graveyard by its effect.
The effect of "Majestic Mech - Senku" that sends itself to the Graveyard only activates during the End Phase of the turn it was Summoned. If it manages to remain on the field longer than that, it will not send itself to the Graveyard during any following End Phase.
If "Skill Drain" is active during the End Phase, it will negate the effects of a "Majestic Mech" so the "Majestic Mech" will not send itself to the Graveyard.
The effect of a "Majestic Mech" to send itself to the Graveyard is a Trigger Effect that starts a chain.
"Majestic Mech - Senku's" first effect is also a Trigger Effect that can be chained to.