darkSpyro

Dark magic is obviously a factor in dotd. So what about it? What caused Spyro and Cynder to fall into it temporarily? What about the dark crystals? How did Malefor activate the Destroyer after taking out his heart?

thru the destroyers backup heart
spyro and cynder have both been expossed to darkness.

what do you mean backup heart?!

To Dark Spyro and Dark Cynder, I thought the same way and put this in a theory:

On Dark Spyro, I probably guessed that to the fact he fell in the Well of Souls(the guide said it), Malefor may have grabbed hold of Spyro and when Ignitus died, you can see from Spyro's face that he's weak. So probably, Malefor took control of Spyro when he was at his weak point.

For Dark Cynder, she was still in the good side TEMPORARILY. She wasen't completely freed from Malefor on that time, so, using his dark influence, Malefor was able to turn Cynder into her dark side.

Dark crystals, dunno but I know about the Destroyer. If you see on the cutscene after the Destroyer, there's dark magic radiating.

Well, the dark crystals were also used to animate the Lava Golem and possibly the Destroyer. They absorb and take away your mana energy for your elements until you destroy them.