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A bug with Slark’s permanent Essence Shift debuff in Dota 2 means that heroes’ agility can be reduced to negative values.
The base agility stat of the hero will never actually drop below zero, but the value is still recorded by the game engine, meaning that any bonus agility will be marked down retroactively. Whether the agility is granted from a level, item, or buff, the hero’s base agility is permanently altered.
Essence Shift also has a temporary stat-stealing aspect. The temporary debuff will never cause a hero to go below zero in any stat, but the permanent debuff isn’t capped.
While most heroes won’t be affected due to the implausibility of the situation, there’s a hero in Dota 2 that has zero base agility and gain: Tiny. This means that any kill on the hero would effectively cause him to “owe” agility to the game engine.
Equipping the hero with a Blade of Agility, which gives 10 agility, is retroactively reduced to nine. As Slark gets more kills on Tiny, the extra agility gain from the item will be eked away as well.
It’s debatable whether this is intended behavior, though. The only other instance of a hero permanently stealing stats from others is Silencer. Any hero that dies in range or is killed by Silencer will lose two intelligence points to Nortrom. In his case, heroes can’t drop below 1.0 base intelligence. If a hero reaches that number, Silence will simply be unable to steal intelligence from the player. The affected hero’s intelligence items will still provide stats and mana to the hero and can’t be affected.
Slark’s permanent agility feature was added to Dota 2 in version 7.20. While the skill has undergone several iterations since then, it seems like the crucial bug has only been recently discovered.
The permanently-stolen agility was initially introduced in version 6.72 in the Frozen Throne mod. It’d steal three agility instead of one and was promptly removed in 6.72b a week later.
This bug might not be game-breaking, but it could be especially annoying for Tiny players. The hero already suffers from extremely low attack and move speed, and stolen agility can greatly affect the hero’s capabilities.
It’s up to Valve whether this is a feature or a bug. The developers will likely try to standardize the treatment of the permanent stat. Considering Silencer’s inclusion in the game for years, it seems more than likely that Nortrom’s way is the correct one.