After doing the regular garden key quest, you can move on to the sanctuary quest from the forrester within the garden in Penumbra.
First you get a mission to destroy 7 different machines by using your Calibration Tuner - Set to Wrong Offset (just use a screwdriver on the calibration tuner to make it, you can buy both tools from Yuttos in Nascense quickly). The missions are near the Ravine mostly, but some locations will take you to the Unredeemed Temple on the northeast side of Penumbra.
Once it's finished you get to the next phase which has 10 steps and it involves killing mobs
1. Calun-Cur - Leader outside the Pen Unred Temple
2. Rawa-El - Marked! outside the Pen Unred Temple
3. Buran-Kuiri - Leader ! outside Pen Unred Temple
4. Chimera Cranii - Marked around 2600x884 in pen (east side)
5. Malah-Ea - Leader! - 1520x1120 in pen (west side)
6. Baran-Kuir - Leader outside the Pen Unred Temple
7. Eron-Cur - Leader outside the Pen Unred Temple
8. Patiarch Prophet Nar Van inside the Pen Unred Temple
9. Omega Carlos Truhillo inside the Pen Unred Temple
10. Unredeemed Vanya inside the Pen Unred Temple
When Vanya dies you're rewarded with your key.
*Where it says UnRed Temple, the island off to the East of the UnRed temple ( the one with the lever to access it ). Also has these mobs