When he passed away, Stengos had himself placed into suspended animation at Tranquil Repose on Necros. It was not like him at all to place himself in suspended animation in the vain hope that someone would find a cure for the organic breakdown of his body.
The Sixth Doctor and Peri Brown arrived on Necros for the funeral, but Stengos' daughter, Natasha, with the help of a doctor friend, Grigory, broke into Tranquil Repose after she couldn't get her father's body from the Courts. She was part of a group that were trying to find out if the Resting Ones were being taken out of suspended animation, and when they opened Stengos' casket, a mannequin was inside.
She later found him: Stengos' head was placed into a Glass Dalek and he was slowly being changed into a Dalek, going in and out of a Dalek mindset as he spoke to her. She killed her father at his request and the Doctor never saw the state he was in. (TV: Revelation of the Daleks)