"Gather these tokens to replenish your waning Glyph energy." - Elder God
Eldritch energy appears in a form of glowing orbs and is either located in certain areas or randomly spawned after devouring enemy souls.
There are two types of Eldritch energy orbs - larger orbs refill 4 and smaller orbs 1 Glyph energy. Smaller orbs are blue with sparkles. Larger orbs have a sparkling purple and white core. The orbs which populate the world will never respawn once collected by Raziel.
Eldritch energy does not feature in the Soul Reaver alphas - effectively making the Glyphs have no cost to use in that version of the game. The earliest build however does feature a glowing orb in a side section of the Silenced Cathedral that may have been a prototype design of the Eldritch energy collectibles.
Although a voice recording describing the Eldritch energy exists, it cannot be triggered in-game.