As you emerge through the portal, you find yourself floating in a formless void. Actually, floating is probably not a good word to use here. You're definitely walking, but there is no floor underneath you, only a swirling mass of gray.
Surveying your surroundings, you see several archways arranged in a large circle. Many are dark and lifeless, nothing but gray stone and emptiness. But there are a few alive with portals swirling within their depths. Each portal has it's own distinct color and some words and/or a symbol carved in the stone above it. These portals are the only ones breathing with life:
Sonic the Hedgehog