The Dreadnought is a fourth era Frostcreature. While it may lack some of the offensive power of other fourth era creatures, it makes up for this by being an incredible defensive unit. It's ability, Pledge of the Giants, dramatically reduces damage taken by surrounding allies, completely negates the vast majority of it, and then sends the remaining damage to the Dreadnought itself. While using Pledge the Dreadnought is also considered out of combat, allowing it to regain health by being near a friendly building.
While a unit is only capable of having Shared Pain on it once, using two Dreadnoughts to cover each other makes for a nearly unbreakable hold. A fully charged, unupgraded Altar of Chaos being set off on two Pledged, unupgraded Dreadnoughts will not come close to killing them.