Aegislash has the ability to change forms during battle, depending on the moves it is using.
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Aegislash is a Pokémon whose body resembles a golden sword. The edge of the blade is white and somewhat serrated near the tip. Where the blade meets the hilt is a single, purple eye with a white slit pupil surrounded by black space. The handle forms a thick, striated spike. Two cloth-like arms emerge from the sides of the hilt. Each arm is primarily black and transitions to a pale purple with a slight swirl pattern at the tip. The sash-like arms split into four tassels at the tips. It always carries an ornate shield in at least one of its arms. The shield is gold and has three brown rings surrounded by black in the middle. It also has a brown rim with three lines radiating toward the center.
Aegislash is capable of changing its stance in the middle of battle using its Ability Stance Change, depending on what moves it uses. It carries out offensive moves in its Blade Forme. In this form, it carries its shield in its left arm and exposes its blade, which it uses to slash its opponents. Defensive moves are performed in its Shield Forme. In this form, it holds its shield up in front of its blade with both of its arms crossed behind the shield. In this form, the centers of the rings on its shield are white. Its Shield Forme can block and decrease any attack. It accomplishs this through a combination of its steel body and a force field made from its spectral power. The Blade Forms can use the strength and weight of its blade to cut through any opponents.
Aegislash is said to be able to detect the innate qualities of leadership; legend has it that whoever it recognizes as a worthy leader is destined to become king. In fact, this Pokémon has attended and served generations of kings. However there a legend where it drained the life of one king, causing his kingdom to fall alongside him. It is also known to use its spectral power to manipulate people and Pokémon alike, and at one point it used its power to create a kingdom to its liking.
Aegislash appears to be based on a possessed sword and shield. Its disposition to sense an owner's potential as leaders and kings, seems to draw inspiration from the ideas of mythical swords of European legend, such as Durendal and Excalibur.
Aegislash may be a combination of Aegis (Zeus's shield in Greek mythology) and slash.
Gillgard may be a combination of Gilgamesh or gild (to cover with a layer of gold), 斬る kiru (to slay with a blade) or kill, and guard.