Lyxen
Great Old One
The only real requirements are (1) one minute to prepare and (2) don't move once you're ready. When preparing an ambush I don't think it's that rare to have 1 minute.
I won't say it's an extremely useful feature, especially for a 10th level one which is a level that a considerable number of tables will never play at anyway, and where you expect features to be, let's say, "high level" whereas that one could be had at a very low level honestly and not imbalance the game at all.
That being said, it's not a totally useless feature, in our groups, the PCs often prepare ambushes (or the NPCs for that matter), and that allows the ranger to position himself in an interesting place and have little likelihood of being found out (+10 is a really a huge bonus).
That being said, @Charlaquin 's suggestion is far from being unreasonable at that level, I understand wanting the rangers non-magical...