"No one has seen an Armored Destroyer since Diablo was exiled from the Burning Hells centuries ago. Before then, they were the bulk of his army, and they crawled thick as locusts over the face of their master's realm. Should the Lord of Terror ever return, I fear they would be his heralds."— Deckard Cain(src)
Armored Destroyers are shock troops only slightly above average in terms of Life pools. They are, however, very quick for their physique, and run swiftly towards the hero, attacking with their bladed arms.
The real danger of these monsters is their 'deep striking' ability, which they often use to gain a dynamic entry. Most of the time, Armored Destroyers are found waiting somewhere above. As soon as a player comes close, they descend quickly, indicated by a golden circle moments before the impact (time given seems to depend on their rarity: normal monsters give 2-3 seconds, whereas elites hardly give a second or two). If they land atop the character, the latter will suffer Knockback (being pushed out of the impact area) and heavy Physical damage. In early versions, this damage could be lethal if attacked by by many Destroyers at once, especially on higher difficulties.
Due to the Destroyers' threat range, even players who have just used a waypoint may be in danger, which is, again, more apparent on higher difficulties. However, the blast radius is small, and Knockback often saves player from multiple hits, so getting away in time will keep character safe.
Unlike other monsters with the ability to descend upon the battlefield, Armored Destroyers are completely immune to damage 1 second after they land, which is exactly the amount of time they need to recover from the impact. Once descended, they cannot ascend again.