re: Magnolia tree help

It seems like some form of fungus and the leaves would be resultant from the fungus. Maybe canker, but I think it more affects branches, not the main trunk. iNaturalist came back with "common tarcrust fungus" (it did get the tree right, too, just from your pic of the trunk), but I haven't found any definitive answer on if it is a problem for magnolias.

Unfortunately, though, if the bark is missing and that black section is growing from the area without bark, it is most likely a fungal infection.

You can search for "treating fungal infections in magnolia trees" and get some info. Mostly it says to try to remove the fungus and treat with a fungicide.