Blamish snail slime is a quest item used in Heroes' Quest. When added to a vial of water with Harralander it makes blamish oil, which can be used on a fishing rod to make an oily fishing rod. The oily fishing rod can then be used to catch a lava eel in the Taverley Dungeon or the Lava Maze.
If you lose your "Blamish snail slime" for some reason, you can go to Mort Myre Swamp, kill a Myre blamish snail, pick up the snail it drops, and mash it up with a Pestle and mortar as long as you have a sample bottle in your inventory. The sample bottle can be obtained from the general store in Canifis. You do not have to speak to Gerrant again after the quest in order to make the Blamish snail slime; just follow the steps outlined above.