Seisyll's own compositions include elegies on the death of Owain Gwynedd and of Iorwerth Drwyndwn grandfather and father respectively of Llywelyn ab Iorwerth.
Seisyll composed his own poems, including laments for the death of Owain Gwynedd and Iorwerth Drwyndwn, who were the ancestors of Llywelyn ab Iorwerth.