Believe it or not, this is a very simple supersaw. Just mess around with saw and square waveforms in various configurations with a whole lot of unison and a little bit of detune. Slap an envelope on the filter to have it close down right after the note hits and mess around with the overall cutoff value while the notes play.

So much of Deadmau5's sound comes from the composition itself, and less from the sound design. If you watch his most recent razer music video on mixing and mastering, he jokes about the secret of the "Deadmau5 Pluck" - an initialized Serum patch with 16 voices of unison and a low pass getting plucked with an envelope.

If you're willing to spend some money, he did release a sample pack of sorts where you can buy a whole much of MIDI files that he's written and never used for songs, so you can pull those chords and arpeggios into your DAW and study directly from his own work.