Martyr is a second-level Life magic spell from Heroes of Might and Magic IV. It costs three mana and, when it is cast on a friendly target, all the damage done to a second friendly target will be transferred to the recipient of the spell. This effect lasts for the entire combat.
What a beautiful and selfless act for someone to surrender their own life so that another can survive!
Similar in concept to the death spell Sacrifice, this can be a very useful spell if used correctly. The martyred stack should be a hero whom you can keep alive with healing spells as transferring damage from one of your creature stacks to another doesn't really achieve very much. If you do 'martyr' a hero, make sure the protected stack doesn't get mobbed too quickly as the hero takes as much damage as the creature stack would otherwise have - their GM combat won't do anything to help.
Many games end with a final battle between two late-game armies, complete with high-level heroes. In these instances, Martyr can be useful to keep your late-game heroes alive in the face of high-level Chaos magic spells and the like, and the loss of some Monks or something is easily justifiable to keep your Grandmaster Tactics hero alive.
Martyr + Summon X = Summoned creatures, being temporary, are the natural choice for cannon fodder.
Martyr + Heavenly Shield = As Heavenly Shield gives you a lot of additional hitpoints, and heroes generally have more than low level creatures, you can use the hero with Heavenly Shield as a Martyr for effective low-level creature defense.