Theoretically speaking, a very small star could orbit a very large planet. Red Dwarfs are stars, and there are very small Red Dwarfs out there (one that we know of being about 15 Jupiter masses), and there are very large planets (non-fusing, non-stellar objects). The largest planet sits in at around 30 Jupiter masses, about double that of the Red Dwarf star named Gliese 229.
And before anyone goes to call me out on the fact that a satellite half the size of its orbital parent would be a big gravitational pull in of itself, technically our own sun doesn't stand still as it orbits the Milky Way Super Black Hole. Earth and the other planets pull on the sun, making them all orbit a point that is not the exact center of the sun.
But, for that planet to be rocky like Runeterra? Very unlikely. But then again, "Magic" is a part of Runeterra. One could argue that the magic of Runeterra has a great gravitational impact on neighboring bodies, potentially grabbing a very small and dim star that the inhabitants of Runeterra mistakenly call a moon. Alternatively, the magic of Runeterra could make a gigantic rocky planet (on the level of 50-100 Jupiter masses) feel like one of something close to Earth's mass, which would be large enough to attract a small star (a brown or red dwarf) to orbit it as it orbits its own star.