[Radek]

You can get out only in a patched game, Morguemaus. The unpatched game suffers from a bug you have just experienced: you must not enter the Pyramid after killing Cet or you will be unable to leave. What's happening exactly?
Once you invoke Cet by touching the portal, the Pyramid starts preventing you from leaving it. There is only one way forward - down with Cet! When you kill Cet, the game teleports you back to Brimloch Roon. But, if the game isn't patched, the game fails to note that you have killed Cet. It's a bug which causes that reentering the Pyramid is your end. The game will continue preventing you from leaving it as if you hadn't killed Cet so far.
If you cannot afford to lose your current game, try to install the patch. Let us hope that you will be able to leave. Another possibility might be the Revert Adventure (not Reset Adventure) button. Quit the game run so that you occur in the town (without the party), enter the inn and click the Revert Adventure button. Your people should be back, well, without the XP and items gained after your last leaving the town. Among others, without the stuff you have just found in the Pyramid. Sorry.