I was referring to the annoyance level of the quest that is apparently in Oblivion.

If Forbidden Legend isn't required for the main story then it was a colossal waste of my time.

Red nirnroot only grows in Skyrim, and was completely unknown in the time of the Oblivion crisis. It only came about due to the events of the Red Year, as an adaptation of the nirnroot plant in order to survive.
There are four kinds of quests in the game.
1) Main quests: These are part of the main storyline which results in Alduin's eventual demise.
2) Civil War: these result in a unified Skyrim, either under the Empire or the Stormcloaks
3) Side quests: Named quests that aren't part of the previous two categories.
4) Miscellaneous quests: unnamed quests that are grouped into Miscellaneous. Some of these may count as side quests for the purpose of achievements, or can lead to a quest of any of the other three types.