If you are specialized in a skill to grant it a new tag like adding poison, it will grant that skill a +level as though it always had that tag. If you can find an example of this not being the case in game, please create a bug report.
Not sure if there are currently ways to make it viable at its current power level, but it definitely seems underpowered in most cases right now and will be buffed in a hotfix.
It would also be good to know people’s opinions on whether we should make changes to the tree that would cause a respec (i.e. anything that makes any existing tree layouts invalid), and how disruptive that would feel mid patch cycle. It’s normally something we avoid where possible in hotfixes, but it would allow more flexibility for tree changes that might otherwise need to wait for 0.8.4. We would still buff the base skill either way.
Tongue of the Aberrant Seer provides +1 to Poison Spell skills, so for a skill to be affected it will need to have the Poison tag and the Spell tag. Aura of Decay doesn’t have the Spell tag, unfortunately.
devtrackers.gg