The AOS stands for Artificial Obscurist Suit. Originally a Cipher prototype, its development was abandoned in favor of the Shadow Pokemon Project. Pi stole it at a young age during an Extras raid on a Cipher Outpost.
It consists of a pair of metal gloves and boots, connected by jointed armature to a central motorpack. It is then possessed by a Rotom.
As it name indicates, it is used to artificially give its wearer access to Obscura. However, it has a horrifying side effect. Prolonged use of the suit causes fragments of Umbrate to form in the wearer's head, leading to madness.
In Pi's case, the Rotom used only compounded the problem. Obitus, the Rotom in question, is a non-verbal killing machine, raised from birth by Cipher for the sole purpose of fueling the AOS.
The combination of the fragments of Umbrate and the murderous nature of Obitus lead to the creation of Madness, a fractional part of Pi's personality bent on taking over his body.