KeyStage is a master instrument controller app for keyboard players, optimized for live performances.
It allows you to control any number of hardware or software synthesizers using any number of MIDI controllers, create keyboard splits and layers, change the functionalities of sliders/knobs/pedals of your MIDI controllers by creating advanced MIDI mappings, view PDF and JPG files (e.g. sheet music) and lyrics, and much more.
In this video all the sounds except the piano sound are produced by synthesizer/sampler apps that are working on iPad in the background, which are:
Jordantron by Wizdom Music
SampleTank by IK Multimedia
iSymphonic Orchestra by Crudbyte
Sunrizer by Beep Street