Damage Threshold for Spelljammers like this one is new, but the concept of damage threshold in Dungeons and Dragons 5e is not. The rules for damage thresholds are listed in the Dungeons Master’s Guide on page 247:
Damage Threshold. Big objects such as castle walls often have extra resilience represented by a damage threshold. An object with a damage threshold has immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet the damage threshold is considered superficial and doesn’t reduce the object’s hit points.Dungeon Master Guide, p 247
This is bad news for PCs that make a lot of low-damage attacks. Your monk won’t be punching a Spelljammer vessel to pieces anytime soon. Your heavy hitters are going to have to bring their A-games, and your dexy friends are going to have to find some heavier weapons.
Damage Threshold for Spelljammers, Among Other Things…
It just so happens my Voivode will be facing down his contestants in the Arena again. One of his favorite means of conveyance is a specially-modified warforged colossus. These fights happen after level 20, and even the colossus is luck to last more than a round or two. My previous vehicles have had overshields that regenerate each turn. My new one will have a damage threshold.
I want to do something a little more interesting, though. I’m thinking that a damage threshhold could be brittle. What if the threshold is removed when the object is reduced to 50% health? Perhaps, certain types of damage bypass the threshold? Maybe subjecting the object to thunder damage removes the threshold altogether?