From Diablo Wiki
Hell Spike is a spell cast by Diablo that traps you in a cage of bones. There are two ways to see if you are affected by this spell. First, and you obviously cannot miss it, you find yourself trapped in the cage where you are stunned. Second, there is a little icon above the "Achievements" icon in the menu bar that appears. Hovering over this icon tells you how many seconds are left before the spell terminates.
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