Sphere of Crushing Doom

From Xanathar's Guide to Everything, page 118.

Complex trap (level 5-10, deadly threat)

The court jester devised a deadly trap to foil anyone who sought to steal his magic fool's cap. The jester's tomb is located at the end of a 10-foot-wide, 150-foot-long hallway that descends sharply from north to south. The entrance to the tomb is a door on the eastern wall at the bottom of the slope, at the south end of the hall.


 * Trigger: This trap activates as soon as the door leading to the jester's coffin is opened. A magic portal opens at the northern end of the hallway and disgorges an enormous steel sphere, which hurtles down the slope. When it reaches the bottom of the slope, a second portal briefly appears and teleports the sphere back to the top of the slope to begin the process again.
 * Initiative: The trap acts on initiative count 10 (but see the dynamic element below).
 * Active Elements: Although the trap is complex in nature, it has a single active element. That's all it needs.
 * Sphere of Crushing Doom (Initiative 10): The trap's active element is a sphere of steel that almost fills the 10-foot width of the hallway and rolls to the bottom of the slope on its turn. Each creature in the sphere's path must make a DC 20 Strength saving throw. On a failed save, a creature takes 22 (4d10) bludgeoning damage and is knocked prone. On a successful save, a creature takes half as much damage and isn't knocked prone. Objects that block the sphere, such as a conjured wall, take maximum damage from the impact.
 * Dynamic Elements: The longer it rolls, the more lethal the sphere becomes.
 * Speed Kills: After its turn, the sphere gains speed, represented by its damage increasing by 11 (2d10). While its damage is 55 (10d10) or greater, it acts on initiative count 20 and 10.
 * Countermeasures: The trap can be neutralized either by stopping the sphere or preventing it from teleporting.
 * Stop the Sphere: Stopping the sphere is the easiest way to disrupt the trap. A Wall of Force can do so easily, as can any object placed in its path that has enough hit points to absorb damage from the sphere without being destroyed.
 * Disrupt the Portals: Either portal can be neutralized with three successful DC 20 Intelligence (Arcana) checks, but the process of analyzing a portal to disrupt it takes time. Faint runes in the ceiling and floor at both ends of the hallway are involved in the functioning of the portals. A creature must first use an action to examine a set of runes, then use a subsequent action to attempt to vandalize the runes. Each successful check reduces the sphere's damage by 11 (2d10), as the disrupted sphere loses speed moving through the failing portal.
 * Alternatively, a set of runes can be disabled with three successful castings of Dispel Magic (DC 19) targeting any of the runes in the set.
 * If the southern portal is destroyed, the sphere slams into the south wall and comes to a halt. It blocks the door to the tomb, but the characters can escape.