If this is a 27" LED Cinema Display whose "hydra" cable has THREE heads (Mini DisplayPort, USB, MagSafe), you need
- A USB-C (male) to Mini DIsplayPort (female) adapter
- Possibly also some sort of USB-C to USB-A adapter, or hub, or dock so that you can plug in the "hydra" cable's USB head and control the display's brightness.
If this is actually a 27" Thunderbolt Display whose "hydra" cable has TWO heads (Thunderbolt, MagSafe), you will need an Apple Thunderbolt 3 to Thunderbolt 2 adapter.
It is important to identify the display correctly, because while the physical connectors are the same, each of these displays has its own signal requirement (DisplayPort or Thunderbolt), and each of these adapters is only designed to translate one type of signal (DisplayPort or Thunderbolt). If you choose an adapter that offers the wrong signal type for the display, you won't get a picture.