This tutorial is not intended only for creating a portal effect, but also to teach you how to use multiple filter effect one after another. Create a new document, 1000x1000 pixels, white background.
Go to Filter > Render > Clouds
Then use Filter > Pixelate > Mezzotint with "Long Strokes" option activated. Your image should look like this:
Now go to Filter > Blur > Radial blur and use these settings: Amount:100, Method: zoom, Quality: best. Your image should look like this:
Now use Filter > Distort > Twirl and set it to 200.
Now duplicate the layer and on the newly created layer go again to Filter > Distort > Twirl but this time apply it with a -400 value. Change the blending mode of this layer to "Lighten". Your image should look like this:
Merge the two layers now. Go again now to Filter > Distort > Twirl and set it to 200. Now press CTRL+U or Go to Image > Adjustments > Hue/Saturation and use any settings you like. I used Colorized, Hue: 200, Saturation: 30, Lightness: 0
