There is a huge ball of metal in the middle of the earth. It spins round and round and that creates the Earth's magnetic field. Unstable currents of magma cause it to shift orientation every now and then, changing the apparent location of the magnetic poles. For a matter of fact, in 2020 the poles drifted further south than ever recorded. There is evidence that the poles flipped several times throughout the history of the earth. Why this happens is unknown. There is debate if the increased surface radiation during these flips (it takes a while, though unknown exactly how long) caused mass extinctions. Read more: https://en.m.wikipedia.org/wiki/Geomagnetic_reversal
In any event, such a magic tower is basically messing with the big spinning ball of metal, possibly changing it's spinning orientation completely. If this magic tower forcefully and permanently anchors the pole to it's location, then the compound of the friction between the core and the mantle and the tidal pull of the moon would possibly rotate the earth, moving the tower from wherever it was to one of the poles. This would take many millions of years, so in the meantime the day/night cycle would be kind of messed up, with more areas experiencing polar winters and summers. Evolution wise, this would favor more seasonal and hibernating animals in a fair portion of the earth, or sea mamels with really long migration routes, like humpback whales.
The other effect of increased friction with the Mantle would be increased volcanic and tectonic activity. Pangea, the ancient supercontinent, broke apart almost 200 million years ago, so 100 million years is enough time for things to move around a bunch, possibly some ridiculously large mountains and volcanoes, but unlikely to significantly change the map. The increased volcanic activity on the other hand, can lead to an increase in extinction level volcano eruptions, known as ve-8 events. These type of events cause ice ages and such, which would change a lot of the evolution of Earth's flora and fauna.
Finally there are the cosmetic changes: an aurora over the tower, compasses pointing towards the tower and birds using the tower as a migration reference.
P.S. If the tower doesn't hold the magnetic field in place, just moved it as a short 1000ish year event, none of this will happen and it will be just a paleontological footnote.