Necromantic Scroll

An extra minion is a flat increase to everything a summoner does, and this hands over a damage bonus on top of it, so it is the largest single jump the event offers that build. Mourning Wood arrives early in the night, but the odds on this drop climb as the waves do, so pushing deeper is worth more than farming easy early kills and restarting the moon.

It folds into the Papyrus Scarab later on with the Hercules Beetle, so do not sell a spare and do not get attached to the accessory layout you build around it.

The Old One's Army accessories look the same on paper but raise the sentry cap instead. Sentries hold a position, which is what you want in the events where enemies walk to a fixed point. Minions travel with you, which is what you want against a single boss or while you are moving through a biome. Most summoners end up carrying both and swapping by situation.