Definition: "Invader"

This is a deletion - look for an indicator word that suggests removing or taking away letters from another word.

'Heartless' is the deletion indicator - take the word for 'giant' and remove the letters from its heart (middle).

Answer: GOTH | Take GOLIATH (giant), remove the middle letters 'lia' to make it 'heartless', giving you GOTH (invader).