Lorne Balfe created the main musical score for Assassin's Creed: Revelations.
Balfe composed the soundtrack for the 2011 game, which follows Ezio Auditore in Constantinople and Altaïr Ibn-La'Ahad in the memory sequences. The score uses orchestral and choral writing to connect Ezio's final major adventure with the earlier origins of the series.
Revelations concludes the principal Ezio trilogy that began with Assassin's Creed II and continued through Brotherhood. Its music therefore had to support both the Ottoman setting of Ezio's story and the historical material involving Altaïr.
Jesper Kyd composed the scores for Assassin's Creed II and Brotherhood, making him the most likely alternative in this question. Balfe's work on Revelations is a separate credit, even though the game retains musical and thematic links with the earlier Ezio titles.