Apparently that file was missing from the old GOG installer together with draining.unity3d. I had the same error. No such problem with the new setup_pillars_of_eternity_2.5.0.11 installer.
Here's the 2.03 spirit_bane.unity3d for people who don't want to redownload the whole installer: http://rghost.net/8f4kGns74
Copy that file to "Pillars of Eternity\PillarsOfEternity_Data\assetbundles\prefabs\objectbundle" if yours is missing.
