Signet of Agility

Signet of Agility costs {2} if you control a Thief creature.
{w}, {t} Sacrifice Signet of Agility: Target creature
gains double strike until end of turn.
{1}{w}, {t}: Target creature gains +1/+1 and
Vigilance until end of turn.
Signets have been around for centuries. They are
almost as old as thieves and assassins.