I'm planning to buy a Godox TT685-N. But I want to use it only as a
slave flash. I'm trying to understand if I need another Godox flash to
be the master or if my current Nikon SB-900 will trigger it.
You do not need another Godox flash to fire the TT685-N off-camera. It can be triggered by several different methods, and only one requires another Godox unit.
"Smart" optical CLS triggering
Your SB-900 (or, if you're shooting a D7x00 or higher end body with a pop-up flash, your pop-up flash) can be used on-camera as a "smart" optical Creative Lighting System (CLS) commander unit, and the TT685-N can be used as a CLS optical slave. This system allows for full control over the off-camera light, with TTL, HSS, and power/group/ratio control. The horizontal lightning bolt button on the right cycles a TT685-N through five different syncing modes:
- on-camera use (green backlight, no icon)
- CLS commander (green backlight, lightning bolt icon)
- CLS slave (orange backlight, lightning bolt icon)
- Godox radio master (green backlight, antenna icon)
- Godox radio slave (orange backlight, antenna icon)
"Dumb" optical S1/S2 triggering
The TT685-N can also, in on-camera syncing mode, be set with the S1/S2 button, to either S1 or S2 "dumb" optical slave mode. This is similar to the SU-4 mode on Nikon speedlights, where any regular (non-CLS) flash burst can remotely fire the flash. But there's no setting communication, TTL, or HSS. S1 will fire on the first burst it sees; S2 on the second burst (to allow for a TTL metering preflash of the master unit).
Godox radio triggering
This is the method that requires another Godox unit.
Neither CLS nor S1/S2 are the preferred method for remotely controlling a TT685-N, because they are optical. Light signals work well in studio conditions where the ambient light level is low, and there are lots of bounce surfaces. Outdoors on location in bright sunlight, optical can lose a lot of range and reliability, and line-of-sight restrictions (the sensor has to "see" the master burst) can become much more stringent. Most prefer to use Godox system's built-in radio triggering system instead. To use the radio signalling, you do need another Godox unit, either another speedlight or a dedicated transmitter unit, like the X2T-N or XPro-N, which will give you more features.