If my opponent has a creature equipped with Darksteel Plate, and I play a Bonds of Mortality enchantment. If I activate the enchantment, does it overrule the Darksteel Plate? Or does this only effect creatures with Indestructible as an ability.
This is covered by the section of the rules titled "Interaction of Continuous Effects".[CR 613]
In short, whenever the game needs information about an object, determine it's current state by starting with the initial characteristics of the object and applying the continuous effects in order of layer. Effects in the same layer (effects that do similar things) are applied in timestamp order (from oldest to newest).
The two effects in question are both applied in the same layer.
613.1f Layer 6: Ability-adding effects, ability-removing effects, and effects that say an object can't have an ability are applied.
As such, they are applied in timestamp order.
613.2. Within layers 1-6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.7.)
The effect created by Darksteel Plate was created first, so it's applied first, then the effect created by Bonds of Mortality is applied second. This means that the creature is "destructible" until the effect from Bonds of Mortality expires at end of turn.
If the creature were to become equipped with Darksteel Plate after Bonds of Mortality resolves, the effect created by Darksteel Plate would have the later timestamp, and it would have Indestructible.
613.6b A continuous effect generated by the resolution of a spell or ability receives a timestamp at the time it's created.
613.6d An Aura, Equipment, or Fortification receives a new timestamp at the time it becomes attached to an object or player.