Frozen Heart's passive should reduce true damage dealt to you by 10%

The entire point of True Damage is that it's not affected by damage reduction/amplification (outside of one very specific case).

You can already itemize against Vayne, Fiora, Darius, Cho'gath and Garen, you just can't reduce their true damage. Did you know that their true damage is a supplement rather than the core of their damage output?

Why Frozen Heart specifically? Would it be a passive for the owner, or an AoE? If AoE, should it protect the owner or just allies in the radius?

Booo you cant make things reduce true dammage because then it is no longer true