Basilisk, also known as Metal Gear Peace Walker or Aurelia, was an AI weapon created by the Peace Sentinels and was infused with The Boss' mindset. It was designed to caculate data flow of an enemy nuclear strike and retaliate likewise by selecting the best target and firing a nuke of its own.
Basilisk Peace Walker

Basilisk becomes Peace Walker

It was also outfitted with a hydrogen bomb that lies inside the sphere that will cause the basilisk to detonate itself in an enemy's homeland if neccesary.