can you use decree of focus when a unit is targetted by realm of death

Yes — if the opponent's Realm of Death is choosing a unit you control, Decree of Focus has a legal target.

Realm of Death is a Fury (and Order) spell, and it targets your unit, so your unit qualifies under Decree of Focus's second clause: "a friendly unit... that's being chosen by an enemy Fury spell." You play Decree as a Reaction to Realm of Death while it's on the chain; Decree resolves first and gives +4 Might, then Realm of Death resolves and applies its -2/+2 before the two units deal damage equal to their (current) Mights to each other, so the +4 is counted in that damage.

Two caveats: it must be an enemy Realm of Death — if you cast it yourself, this clause doesn't apply. And if the target stops being chosen by that spell before Decree resolves, Decree mistargets and the +4 is lost (Rules 359.3.e.2, 359.3.e.5).