Easy quest. Must complete Kalecgos's quest first which you'll start/acquire in the Chamber. You can GET the 'Unlocking the Power' quest at the same time, but Magni won't do anything until Kalecgos's is completed first.
Kalecgos's quest will take you to the top of The Nexus in Coldarra in Northrend's Borean Tundra. Speak to Kalecgos's to start the quest, fly to the center of the crystal ball in front of him after he's finished talking and grab the Dragon Scale. This is an upgrade for your Crucible of Flame in your Heart of Azeroth which will upgrade it to level 4. Once you grab the scale, talk to Kalecgos again and he will return you to the Chamber. Magni will then have a quest which is just a cinematic and all you get from that is a Toy Heart of Azeroth. Infuse the Heart with the dragon scale and poof...all done. Acquired a toy, and level 4 Crucible of Flame.