encore

PS - if you don't already have such a serial number from a past purchase, you can't buy one now

An Encore alternative http://tmpgenc.pegasys-inc.com/en/product/taw6.html

Encore is bundled with Premiere Pro... you must have a serial number for a product that includes Premiere Pro

For CS6 and earlier you may download ONLY if you bought direct from Adobe via download
-Log-in to your account https://account.adobe.com/products
--then click Order history on the left and the link to your order that shows on the right
--and finally click the link "Downloads and serial numbers" that will appear
-if you did not buy direct from Adobe via download you must find your original install
--if you have an Education or volume license you MUST use your original media
--the link for an Education or volume license download is not available at all
-if you bought a disc and don't have a drive you will need to buy an external USB drive
-if you bought a download from a 3rd party vendor you will need to contact that vendor
More download information https://helpx.adobe.com/x-productkb/global/find-downloaded-file-app.html

Encore was in CS6.  Do you have an active license for CS6?