Master strike 8th edition warhammer fantasy battle vampire counts vampiric power overview
Master strike is an entirely new vampiric power in 8th edition warhammer fantasy battle and also makes use of one of the new special rules that previously did not exsist before in 7th edition. There is no precedent from previous vampiric powers for master strike except perhaps the now retired magic item the sword of kings (which was always more effective for wight kings anyway) that granted the wielder the killing blow special rule.
Master strike is a cheap vampiric power available to vampire heroes, vampire lords and Strigoi ghoul kings it allows them to sacrifice all their attacks in order to make a single attack that has the heroic killing blow special rule. The heroic killing blow special rule has the chance to kill outright any model of any type regardless of toughness, wounds, armour saves and regeneration (ward saves can still be made) on a to wound roll of a natural 6.
Pros and cons of master strike
Pros for master strike are that it is cheap for a vampiric power and can give an otherwise poorly equipped vampire the chance to kill outright a foe it may otherwise struggle to even harm. Empire steam tanks make an excellent choice for using master strike against as they are automatically hit in close combat and the vampire with this vampiric power can hide in a unit allowing them to take the brunt of its damage. Other good targets include high toughness models such as the tomb kings war sphinx.
Cons are basically that both vampire heroes and vampire lords have a lot of high strength attacks already that can be further enhance with basic equipment such as a lance, magical weapons or the potion of strength. A vampire hero can easily muster 4 strength 7 attacks on the charge armed only with a lance. Sacrificing this for 1 attack with the heroic killing blow special rule is only the best option in a limited number of circumstances against a limited number of foes.
Remember as well that you must first also roll to hit and then roll a 6 to strike with heroic killing blow. Once your opponent knows you have this vampiric power (and he survives of course) he is then likely to direct his attacks against your vampire to prevent this happening.