OpenTomb is a cross-platform reimplementation of classic Tomb Raider 1—5 engines that were developed by now defunct Core Design. It is not a patcher or DLL extension of already existing engines.
The project does not use any old Tomb Raider source code, because all attempts to retrieve sources from Eidos / Core were in vain, despite many petitions by the community. Instead, everything is being developed completely from scratch.