Apprentice's Scarf
Increase your max number of sentries by 1 Increases summon damage by 10%
3Dropped by
why it is strong
An extra sentry slot plus a flat summon damage boost, dropped at a point in the game where offensive accessories barely exist. For a summoner that is close to a free tier upgrade. For anyone else it is still one of the very few pre Hardmode accessories that adds damage, and a second sentry parked in your arena works no matter what you are holding.
the four do not stack
The Huntress's Buckler, Monk's Belt and Squire's Shield carry the same pair of bonuses, and wearing more than one of them gains you nothing. Pick whichever one has the class effect you want and put something else in the other slot.
getting one
The Dark Mage in the opening tier of the Old One's Army drops it, and Expert improves the odds. That tier is cheap to run again and again, so if you want the scarf you will have it after a handful of attempts rather than a week of grinding.