Miner Magnus is a subject of Miscellania, which you need support from in Throne of Miscellania. He is located in the coal mine. When mining at the mine of Miscellania, all of your ores (and gems) are given by you to Miner Magnus, which will, if Throne of Miscellania is completed, raise your popularity in your kingdom by .8% percent. Strangely, this NPC uses the old mining animation and will continue to mine from the rock even if a player mines the ore from it and gives it to Magnus. Miner Magnus is one of four residents whom you can assist to gain favour; the other three are Gardener Gunnhild, Lumberjack Leif, and Fisherman Frodi.
Miner Magnus is Latin for Big Miner. Magnus is also a Scandinavian forename, and as such his given name fits with the Scandinavian theme of the Fremennik province.
Miner Magnus appears to be a terrible miner, as no matter how long you watch, he will never gather together even a single ore without your help.
If you attempt to mine the coal rock that Miner Magnus perpetually works on, the chatbox will state Miner Magnus is already mining that.