Disk encrypted
How to erase encrypted disk
How to erase encrypted disk
Is this your boot disk? If so, then you would first boot into the "Recovery" Partition, and using Disk Utility there, click on erase to create a new file system. There is no real need to zero anything or us any security erase pattern, as all the data is encrypted, and creating the new file system will destroy the decryption key.
You will have to unencrypt it first.
Disk encrypted