In one of the larger rooms in Vorador's Mansion, you can see a large painting. It is a portrait of a woman's face. However, the interesting thing is that she may be a reference to Umah, the female vampire from Blood Omen. If you look closely at her forehead and the sides of her face, you can make out tattoos similar to Umah's.
In fact, one event that was removed from the game was the appearance of a pre-vampiric Umah.
Tony Jay, who provides the Elder God's voice, often played the roles of villains, due to his deep baritone voice, in animation and video games. His roles included the virus Megabyte in the 3D cartoon ReBoot, the immortal Transcendent One in Planescape:Torment and the Narrator in The Bard's Tale(2004). He also voiced Zephon in Soul Reaver and Mortanius in Blood Omen( in Defiance, he is played by Alastair Duncan). Sadly, Tony Jay died in August 13,2006.
There is a cheat code that replaces Kain's weapon with a cardboard tube, with Kain uttering "Fear the Tube". This is a reference to the popular web comic Penny Arcade and its "Cardboard Tube Samurai" character.
The boss monster Turel, who Raziel encounters later in the game, was cut from the very first Soul Reaver. Although Turel was mentioned in Soul Reaver 1 as one of Kain's vampire clan leaders and his disciples could be found in the game, he himself was missing without any plot-related explanation. Turel's appearance in Defiance adds to the series' brilliantly constructed time-travel plot. Furthermore, a strategy exploiting Turel's weakness is needed to defeat him in Defiance, similar to the bosses in Soul Reaver 1.