I had a situation to develop a custom login screen for custom claims based authentication. For that, we have to inherit the SignIn page from FormsSignInPage class (from Microsoft.SharePoint.IdentityModel.dll). We normally refer the SharePoint development related DLLs from ISAPI folder and But I unable to find this dll in SharePoint 2013 development environment.
1. Open the Windows Explorer
2. Navigate to C:\Windows\Microsoft.NET\assembly\GAC_MSIL\ Microsoft.SharePoint.IdentityModel
3. And you can find the dll here C:\Windows\Microsoft.NET\assembly\GAC_MSIL\ Microsoft.SharePoint.IdentityModel\v4.0_18.104.22.168__71e9bce111e9429c