To get access to the abyss you need to do the mini-quest with the Mage of Zamorak (map on right).
The abyss is a location where you have access to all the altars in Runescape at one place.
Is it recommended you get some Pouches
so you can carry extra Rune Essence.
Go to Edgville
, fill up with Rune Essence and go to the Mage of Zamorak who will send you to the abyss, use your Nature Tiara
to get the the altar and craft your runes.
Then use a Amulet of Glory
to teleport back to Edgeville. Repeat this process.
From level 44 - 99 you will need to buy 1442,087
Rune Essence which will cost: NA
But, you will turn the Essence into Nature runes, you can sell Nature Runes as you make them (so you dont run out of cash).
Selling 1442,087 Nature Runes will give you a MASSIVE profit of: 98M