Fortify's 10% damage reduction only applies to the slice of an incoming hit that would push your current Life to or below your current Fortify amount - full-Life characters get little benefit from Fortify against the FIRST big hit of an engagement, but massive benefit once Life has dropped near the Fortify band.
This is the single most important non-obvious fact for understanding why a character with a large Fortify pool can still occasionally take a suspiciously large hit early in a fight, then become nearly unkillable moments later once Life has dropped - it is the same mechanic behaving very differently depending on current Life state.
See snap_fortify_does_not_snapshot for the exact per-hit overflow formula; this entry restates the same mechanic from a pure defensive-planning angle.
Worked example: 1,000 Life / 500 Fortify, hit for 600 -> ends at 410 Life (only the final 100 of the 600 damage got the 10% cut).
Don't rely on Fortify alone against a single opening burst while at full Life; pair it with a Barrier (which mitigates from the top of your Life bar down, see def_barrier_depletes_before_life) to cover the gap Fortify doesn't protect.
defensiveConfirmedBarbarianDruidNecromancerPaladinWarlock
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