Derevi, Empyrial Tactician is banned as a commander in Duel Commander format, but it may be part of your deck.

You can activate Derevi's last ability only when it is in the command zone.

When you activate Derevi's last ability, you're not casting Derevi as a spell. The ability can't be countered by something that counters only spells. The ability isn't subject to the commander tax, nor does it increase the tax if you cast Derevi from the command zone later in the game.