The Battle of Najera was arguably the peak of the Black Prince's military career. England allied with Peter I against his half-brother and decisively defeated the Trastamara, but Henry escaped, the alliance broke down, and Peter was eventually overthrown by Henry, who fled across the Pyrenees and then eventually returned to Castille with French backing. Castille's subsequent alliance with France was a major contributing factor to the rapid collapse of England's position in Gascony, since Castille had a powerful navy; the Castillan fleet's victory at the Battle of La Rochelle was perhaps the most immediate consequence of Henry's ultimate victory over Peter.
So what if Henry was unable to escape and was captured and then executed in the battle? Peter I was known as "the Cruel" to history, but some have suggested that he was merely attempting to centralize power against the nobility. Indeed, Castille would be plagued by a weak monarchy in the subsequent century- it was not until the reign of Ferdinand and Isabella that Castille was finally able to curtail (though not extinguish) the power of its aristocracy. So Peter's continued reign may or may not be able to partially offset this historical trend.
So what if Henry was unable to escape and was captured and then executed in the battle? Peter I was known as "the Cruel" to history, but some have suggested that he was merely attempting to centralize power against the nobility. Indeed, Castille would be plagued by a weak monarchy in the subsequent century- it was not until the reign of Ferdinand and Isabella that Castille was finally able to curtail (though not extinguish) the power of its aristocracy. So Peter's continued reign may or may not be able to partially offset this historical trend.