The Destroyer is the Hard mode version of the Eater of Words. He is a giant mechanical worm as opposed to a giant fleshy worm. It can only be summoned by using a Mechanical Worm anywhere.
NOTE: The Mechanical Bosses can only be summoned after dusk (7:30PM) and they must be destroyed before dawn (4:30 AM). It is also wise to spend the time making an Arena as a place to fight the Hard Mode Bosses, especially if you going after them solo. You will also want to utilize potions and buffs, such as Ironskin, regeneration, Thorns, etc. to make the battle easier on you.
RECIPE: - craft at an Orichalum/Mythril anvil
7 Souls of Night
6 Rotten Chunks
5 Copper Bars
5 Iron Bars.
1. You do not need to be in the corruption to spawn it (unlike The Eater of Worlds).
2. It summons probes that attack you, but they can be farmed for hearts/health
3. It burrows through the ground, so your best bet is to fight it in an arena or a large open area to maximize hitting multiple segments. That also means you should spawn it away from your NPC area if you want them to survive.
4. Use a weapon that penetrates, such as a flail, spear, Death Sickle, or magic spells in order to hit multiple segments with each attack
5. Wear sturdy armor (Turtle preferred) since it is large and you will get hit a lot.Also might be wise to wear a Cobalt/Ahnk Shield
6. Look out for the "The Jungle grows restless" message, as this usually occurs after killing a Mechanical Boss for the first time. This means Plantera bulbs will spawn in your Underground Jungle.