The reference itself can be found in The Sacrifice of Ruanon and isn't explained further:
There are ninety-one priests in all, Lone Wolf, said his Kai voice
inside him. It seemed to have been a long time since he had heard its
dustily cool tones. The number ninety-one was that of the Darklord
Vashna, who chose it because of its factors, seven and thirteen –
seven to signify the everlasting death-in-life and life-in-death that
he possesses, and thirteen to denote the power of the Evil at his
These are the aspects of Naar that he was able to call upon to serve
his will while he walked upon Magnamund; they will become his slaves
once more should the Darklord be reborn.
The sole reference within either the novels or the gamebooks that connects the numbers seven and thirteen is from War of the Wizards. There are 13 magical powers in total, of which the first seven are described as the "lesser magicks".
There are thirteen Magical Powers, the first seven of which are called
the Lesser Magicks
- Sorcery, Enchantment, Elementalism, Alchemy, Prophecy, Psychomancy, Evocation.
The Higher Magicks are:
- Thaumaturgy, Telergy, Physiurgy, Theurgy, Visionary, Necromancy
The implication seems to be that unlike the player (who must select their abilities from the list) Vashna has mastery of all of the types of magick, both the seven and the thirteen.