If “Blade Knight” destroys “Spear Cretin”, “Spear Cretin”’s effect is not applied.
When a card that selects a card in the Graveyard as a target, such as “Monster Reborn”, “Mask of Darkness”, “Magician of Faith”, “Spear Cretin”, or “The Shallow Grave”, is activated, you may chain “Disappear” to remove the targeted card from play, and the “Monster Reborn”, etc., will resolve without effect. This is because these cards select their target at activation, not resolution.
If you Flip Summon “Spear Cretin” while “Skill Drain” is active, “Spear Cretin”’s effect is negated when it is sent to the Graveyard because it is a Flip Effect and “Skill Drain” negates Flip Effects.
If “The End of Anubis” is face-up on the field when a flipped “Spear Cretin” is sent to the Graveyard, then “Spear Cretin’s” effect does not activate. But if “The End of Anubis” was merely on the field when “Spear Cretin” was flipped, but is NOT on the field when “Spear Cretin” is sent to the Graveyard, then “Spear Cretin’s” effect DOES activate when sent to the Graveyard because “The End of Anubis” is no longer on the field to negate it.
Netrep Rulings:
“Spear Cretin’s” effect is optional.
A player may not choose the same “Spear Cretin” that was destroyed as a target for its effect.
A monster destroyed at the same time as “Spear Cretin” may not be chosen as the Special Summon for the effect of “Spear Cretin.”