If Megamorph and Pestilence are equipped to the same monster, the one that was equipped most recently will be applied. If that card is later removed from the field, the first Equip Spell’s effect will be applied.
If you use Shrink on a monster equipped with Pestilence, the monster’s ATK will be half of its original ATK while Shrink is being applied. It will return to 0 when Shrink is no longer applied.
If Pestilence is equipped to a monster that is already being affected by Shrink, its ATK will be 0. If Pestilence is removed from the field before Shrink’s effect stops applying, the ATK will become half of its original ATK.
If Inverse Universe is applied to a monster equipped with Pestilence, the ATK and DEF switch and the monster’s DEF becomes 0. If Pestilence is equipped after Inverse Universe is applied, the monster’s ATK will be 0.
If Pestilence is equipped to Command Knight, its ATK will first become 0 and then become boosted to 400 by the effect of Command Knight.
While equipped with Pestilence, the ATK of Fortune Lady Light will be 0 regardless of its Level and any changes to its Level. If Pestilence is no longer equipped, Fortune Lady Light’s ATK and DEF will return to 200 x its Level.