Beware: caution must be taken not only getting to the arena but also during the miniquest. PKers are able to attack you while you are doing the minigame fight
Players with level 60 or higher Magic can begin The Mage Arena. Head out into level 56 Wilderness to start, but watch out for player-killers. The quickest way to get there is by use of the levers in the ruins of Edgeville and in the small building near the gate to West Ardougne. Pulling either of these levers will teleport you to the Deserted Keep, a small island surrounded by lava in level 54 Wilderness. From there, run north, slash a web with a knife, sword, or other sharp object, and then proceed west a short distance to the Mage Arena bank, which also requires players to slash webs to access.
North-west of the actual Mage Arena is a little broken down house. Use a knife or a weapon which has the 'slash' attack style to cut down the webs in the house. Once the two webs have been cut, pull the lever on the wall of the house to be teleported into a small cave. While you are in here, you are completely safe from PKers. In here, is a rune store, a bank, and Kolodion, the non-player character who must be talked to in order to start the Mage Arena minigame. Kolodion will also be fought during the minigame, so just talk to him to begin. After a long dialogue, you will end up fighting him.
Kolodion changes form as the fight progresses. At the start, he appears in the form you see him when you speak to him. He can only be damaged using Magic. After defeating this form, he transforms into an ogre-type creature. Despite being in Melee range, this form and all forms afterwards are magic-based, so the Protect from Magic prayer will make you invulnerable to his attacks. The next form is a giant frost spider, then a ghost, then finally a level 112 black demon.
For all of these battles, it is recommended to use the strongest spell you have the level for. As each form changes, the Hitpoints of the form increases. With Prayer and decent food, the battles are fairly easy to win. At level 70+ Magic using Ahrim's armour set, around only 100 casts of the Wave spells are needed to defeat all five of Kolodions forms. Iban Blast is also a viable option as its power allows for Kolodion's defeat in as little as 50 casts, even at level 60 Magic wearing only Mystic robes.
You can exit the fight at any time by running out of the arena and pulling the lever to the north. Kolodion will be in the form you left off at when you return.
Once Kolodion is defeated, the minigame ends and players will be teleported out of the arena, and Kolodion will congratulate them on a fight well fought. He will then ask players to choose their God. To do this, head into the nearby 'Sparkling pool' and you will come out near a staff merchant. Head past him into the room with the three god statues. Now, choose your god and pray at the relevant statue. Players will then receive one of the God capes, determined by which god is prayed to. After receiving the cape, head back and speak to the merchant to receive one of the God staves; which will be the partner to the cape. To unlock the god spells and use them outside of the arena, they must be cast 100 times inside the arena. It may be prudent to only attack the Guthix and Zamorak-aligned battlemages, as some player killers will dress up as a Saradomin mage and hop worlds in order to skull-trick players into losing all of their items. Also note, due to having to cast the spells manually, hiding the attack option won't prevent getting tricked. To obtain other god staves to cast other god spells outside the arena, talk to the merchant who will charge 80,000 coins as a fee for the staff and then cast the other spell 100 times in the arena. God staves cannot be wielded with the cape of another god, for example, the Saradomin cape may not be worn while wielding a Guthix staff. The capes are required to cast the Charge spell at level 80+ Magic, and can be changed by praying at the desired god's statue.
The staves are shown below.