Shift at Will was an ability that Raziel was intended to have gained in the original version of Legacy of Kain: Soul Reaver. An upgrade of the Shift Glyph, the ability would have allowed Raziel to move from the Spectral to the Material Realm without the aid of Planar Portals. Raziel is believed to have gained this ability after a "partial victory" over Kain in the Chronoplast. With the removal of many of the post-Chronoplast events from the original Soul Reaver storyline (and the storyline changed so Kain travelled into the past at this point), the ability was cut from the final game.
Shift at Will would have enabled Raziel to more easily traverse the Spectral and Material Realms of Nosgoth, allowing him to reach later areas of the game that were deprived of the planar portals (which up to this point were vital to shifting realms). Primarily this ability would have enabled Raziel to switch Realms at will, without requiring planar portals - although when shifting from Spectral to the Material Realms, a full Soul Spiral would still be required. This mechanic would have 'opened up' many of the later areas (such as Undercity, Temple and Turel's Clan Territory) as well as many bonus areas. The ability can still be utilised in Soul Reaver through the use of button cheats or the debug menu, though many of the areas it was originally intended to have reached have been removed completely, so it has little function apart from enabling the user to solve 'glyph-shifting' puzzles with less effort.
The Shift at Will ability allows Raziel to perform a number of interesting effects allowing him to appear and disappear at will and allowing (with the special properties of the Spectral Realm) a form of 'pseudo-teleportation'. Any human which witnesses Raziel performing these abilities will be suitably horrified.
It is unclear what exactly the 'partial victory' over Kain would have entailed, but given that most of the abilities in Soul Reaver require the consumption of a Soul, it may be that Raziel may have been able to consume part of Kain's soul, possibly in a manner similar to the 'soul sipping' he can use (non-fatally) on humans in Soul Reaver . Video reconstruction of this event can be found here.
Developer interviews suggest that the Force and Sound Glyphs were originally intended to be collected after Raziel gained the Shift at Will ability. Presumably Planar portal were put back into these areas after the Shift at Will ability was removed. The Sound Reaver was also to have been gained at this stage, but it was removed altogether.