Buy OSRS Slayer's respite(m)
Slayer's respite(m) is the mature version of slayer's respite in Old School RuneScape. It is an ale that boosts your Slayer level by 4 for a short time. The regular version only boosts Slayer by 2, so the mature ale gives a bigger lift. Drinking it also heals 1 Hitpoint. As a trade off, it lowers your Attack and Strength by 2 levels.
A higher Slayer level can let you take on tasks or monsters just above your current level. The boost does not last long, so drink the ale right before you need it.
Slayer's respite(m) is a members item. It is tradeable and noteable, but it cannot be equipped or stacked. It weighs 0.55 kg.
Making this ale yourself takes work. You need level 59 Cooking and a brewery, like the vats in eastern Keldagrim or Port Phasmatys. Each batch has only a small chance to turn out mature, and brewing takes one or more days. Buying the finished ale skips the long wait and the low odds. It is a simple way to get a Slayer boost when you need one.