Aegislash is based on a sword and a shield, which were very common weapons used by knights in combat during medieval times.
It could also have been influenced by mythical swords of European legend, primarily Excalibur of Arthurian legend.
Aegislash has received generally positive feedback from fans when Pokémon X & Y were first released. It has a unique Ability called "Stance Change", which enables it to switch it's mode of combat.
To make great use out of this Ability, Aegislash is capable of learning King's Shield, which enables it to change between Shield Form and Blade Form.
Blade Form has high physical Attack and Special Attack, while Shield Form has high physical Defense and Sp. Def. With it's Steel/Ghost Type combination, this allows Aegislash to be a potent threat in battle.
Ever since X & Y's early days, Aegislash has remained in the Uber tier due to it's amazing Ability, admirable Type combination, and the raw power to back up it's viability.
Aegislash can run a few different sets that would encourage the use of Trick Room to enable it to outrun a lot of fast Pokémon in the metagame. It can be a dangerous attacker by using Swords Dance to raise it's already good physical Attack and then utilize Shadow Sneak for reliable priority.
Aegislash still has some drawbacks in spite of it's competitive godsends. It can easily get overwhelmed by extremely powerful Pokémon, such as Primal Groudon and Yveltal. Furthermore, it's viable offensive movepool is limited and can easily get shut down by Will-O-Wisp if it runs a physical attacking set. It also has no reliable recovery outside of Leftovers, which can easily get removed by Knock Off.