You can regulate it straight to 12V with just the triac. Just make sure you turn it on when the AC voltage wave is 12V approaching 0V (e.g. approaching pi and/or 2pi radians for a sine wave), otherwise you'll get the full 500V into your circuit if the triac latches on for more of the cycle. You could alternately use a transistor (FET or BJT is OK), which doesn't have the latching characteristic.
Disclaimer: Personally, I wouldn't recommend you doing this as it is potentially lethal. The output voltage will also not be isolated.