Mercury Treads is reducing the effectiveness of Brittle.

So, after much testing, Brittle does the following:

1. If the target has Mercury's Treads: -21% Tenacity.
2. If the target doesn't have Mercury's Treads: -30% Tenacity.

The reduction in Tenacity is additive and occurs after all sources of positive tenacity (which is multiplicitive).

• 0% Tenacity -> -30% Tenacity
• 10% Tenacity -> -20% Tenacity (Rank-1 Burning Agony)
• 15% Tenacity -> -15% Tenacity (Swiftness Mastery)
• 30% Tenacity -> 0% Tenacity (Rank-5 Burning Agony)
• 30% Tenacity -> 9% Tenacity (Mercury Treads)
• 40% Tenacity -> 10% Tenacity (Rank-5 Burning Agony + Swiftness Mastery)
• 51% Tenacity -> 30% Tenacity (All 3)

It appears as though Mercury's Treads is reducing the effectiveness of Brittle (30 * 0.7 = 21), despite no other sources of tenacity doing this.

I cannot easily test if this is just a visual bug, or whether it's a mechanical bug.

Just press C to see your extended stats while affected by Brittle vs not affected by Brittle while you have Mercury's Treads or don't have Mercury's Treads.

UPDATE:

With 2 stacks of Brittle (yes, it stacks):

1. No Mercury's Treads: -60%
2. With Mercury's Treads: -42%