It's a standalone mod (I think you just need to own HL2 for licensing reasons, but you don't need it actually installed), the story and levels are original (although they tie into both Half-Life and Portal lore - you're a Combine guy hunting down Mossman). It's pretty great, genuinely retail game quality despite being a free mod.
Is this a remake like black Mesa?
It's a standalone mod (I think you just need to own HL2 for licensing reasons, but you don't need it actually installed), the story and levels are original (although they tie into both Half-Life and Portal lore - you're a Combine guy hunting down Mossman). It's pretty great, genuinely retail game quality despite being a free mod.