User blog comment:Emptylord/Custom Champion - Behemoth the Earthbreaker/@comment-27802145-20170430041627/@comment-7709681-20170430205325

Stacks generate above the HUD, and a visual queue on the model to be visualized how the ability is charged (for instance, you describe him as a sentient volcano, then give him a magma-core that grows in bright the closer to the threshold, and fully irradiates when preparing to detonate).

The stacks begin to generate from the moment he receives the first instance of damage since the last detonation until the ability detonates (effectively tracking damage taken after the threshold is reached). If for Camille there's a hidden damage type calculator do determine which shield type should be used against an opponent, I don't see why Behemoth cannot.