In Soul Reaver, Raziel had to expend Eldritch energy to cast Glyph spells. Powerful spells required more energy than the weaker ones; only the Shift Glyph was free to use.
After Raziel claimed his first Glyph (excluding Shift), he could collect limited amounts of Eldritch energy from the environment. Sometimes it was left behind by defeated enemies. A small ball was worth one point; larger balls of energy were worth up to five points. Raziel could also top up his Eldritch energy stock at special 'Eldritch Energy Recharge' locations - marked by a blue eye - usually found at Glyph altars.
Raziel's capacity to carry Eldritch energy increased with each Glyph he found. He could raise that capacity artificially (to a maximum of fifty-two points) by finding the rare Eldritch upgrade artefacts. There were only five of these artefacts in the game.
It can be noted that the term 'Eldritch energy' appeared in Prima's Official Soul Reaver Strategy Guide and the US version of the Soul Reaver manual. The UK manual simply called it: 'Glyph energy'.