i think it's kind of interesting to consider the dalish's perception by outsiders, bc it's like:
from the dalish's (accurately cautious) perspective, if their campsites or common migration routes get discovered, they're doomed, and will immediately get cornered by the chantry or the nearest noble who dislikes elves. so they're incredibly hard to find to begin with, and if they do get found, it's in their best interest to leave zero survivors and then vanish immediately.
plus, they have slightly better night vision on a physical level, as well as being well versed in outdoor stealth from a very young age. plus they have mages. plus they're very frugal and don't have any real material goods. and they would be very averse to anything like a "fair fight" bc it's not like they have... soldiers who are actually expendable. anyone who gets hurt or killed is meant to be doing another job as well to keep the clan running, and is a beloved small community member as well.
so i think a lot of their Terrifying Reputation is bc they do (seemingly) completely unprovoked attacks that can look like this:
some human expedition goes out into the wilderness for logging/hunting/fishing/herb gathering/bald/nuance, sees zero sign of any dalish around
suddenly, they all get completely massacred, at night, with no warning or mercy shown, and with half the dead creepily strangled by tree roots or smth, and their entire campsite picked over for valuables. which looks like theft was the motive, but in an extremely ruthless and unnecessarily cruel way.
when a revenge party is sent out, there's still no sign of the dalish, aside from an abandoned campsite and no clear tracks leading anywhere
of course, if any of these steps go badly... it leads to the clan actually getting hunted down and decimated by mercenaries or templars or chevaliers or whatever. i think ralaferin (the codex entry clan) and lavellan (lavellan) are some of the few clans that have intentional exposure to outsiders at all. imo, they're numerous/magically protected/well-armed/prosperous/connected/bald/nuanced enough that they're aware it'll be an Ordeal for any local noble to run them out. so they're willing to take more risks, like talking to scholars and giving them codex entries, or sending out mages to obscure dangerous missions (both mage lavellan, AND two dalish mages from clan ralaferin who are in the dai multiplayer characters), or ending up enmeshed in wycome's city council or whatever.