Elemental Fonts were scattered across Nosgoth in Soul Reaver 2. Initially dormant, these basins were linked to the four elemental Reaver Forges in that game.
Whenever Raziel activated a Reaver Forge in Soul Reaver 2, the many fonts attuned to that forge's element would awaken, and expel a stream of elemental energy (Dark, Light, Air or Fire). Raziel could imbue the Soul Reaver by bathing it in this energy, and once imbued, the wraith-blade would retain its elemental affinity until Raziel entered the Spectral Realm. The fire enhancement was not lost when Raziel was submerged in water, unlike in Soul Reaver, where the Fire Reaver - gained from the Fire Forge [SR1] - was extinguished.
The fonts were often guarded by Shades, who would steal the energy and hold on to it. Raziel had to fight and defeat them before he could use the font. The fonts also had different markings, helping Raziel to distinguish one elemental type from another.
Raziel did not use Elemental Fonts in Defiance; they didn't exist. There was originally an intention explain their disappearance in-game. It would have been implied that the Elder God had destroyed them post-Soul Reaver 2, presumably to rob Raziel of his elemental Reavers at the source.
Luckily, the loss didn't cause Raziel any long-term difficulties. The Reaver upgrades he gained in Defiance were more permanent than their Soul Reaver 2 predecessors; he could switch between their elemental affinities at will. No fonts were needed.
- [SR2] Raziel: "Now I understood the purpose of the mysterious basins I had seen throughout my journey. They were elemental fonts, each attuned to a fundamental essence. When I forged the blade, every font in Nosgoth with the same elemental attunement would simultaneously quicken.
The Reaver's enhancements were never permanent, but these fonts enabled me to re-imbue the blade by bathing it in the elemental energy distilled therein."
- The first Defiance Q&A page at The Lost Worlds