It is only plagiarism if you try and pass off somebody else's work as your own. Hydras that regrow heads when slain and using primary and basic secondary colours to denote elements are not exactly original; those stories have been around for thousands of years.
Meh, Blizzard references popular games all the time. This one is clearly reachiing though. An unknown game with a hydra and a cave doesn't a reference make.
A nice little homage at most, and I don't see why people would have any issues with it even if it was directly based on that game.
It's not like there's a whole lot of truly new ideas and concepts out there, not only for games. But for anything really, almost everything is based on something that came before anyway.