1tsp.bourbon vanilla extract(regular vanilla works too)
Place all ingredients into a pot.
Bring to a boil.
Reduce heat to a simmer and cook the liquid down by about three fourths.
When it's done, you won't see much liquid left. Just the rhubarb. It will thicken up when chilled.
Transfer to a food-safe container and chill in the fridge until ready to use.
Note that you will want to warm this up again to serve it. Particularly if you use erythritol. If you intend to use it more like jam, I recommend a different low carb sweetener of your choosing to replace the erythritol completely.