What video game published by Sierra Entertainment includes an antagonist figure who's mystery is the core of the series? 
Alma Wade: Alma Wade is a major antagonist and key figure in the "F.E.A.R." series of first-person shooter horror video games by Monolith Productions, introduced in "F.E.A.R." in 2005. Alma Wade:  The mystery of Alma is the very core of the series. F.E.A.R.: F.E.A.R. First Encounter Assault Recon is a survival horror first-person shooter video game developed by Monolith Productions and published by Sierra Entertainment.
F.E.A.R.