As Duarte already assumed, your mesh has uneven density. This is a problem for all sculpting methods, since the algorithms get confused about which edges to subdivide.
However, there is a possibility to fix this by using Dynamic Topology and relative detail.
You also want to use a "sharp brush" Texture in form of a simple white png with a black outline (image below).
Fixing the Topology and smoothing the Surface
In Sculpt mode, activate Dyntopo, switch from "Constant Detail" to "Relative Detail", change the Detail Value to 4.0 and activate "Smooth Shading".
While you are here, also deactivate all settings in the Symetry/Lock section below (image below).
Then, select the "Clay Strips" Brush, lower its strenght to 0.1 and lock it to "Area Plane".
Under "Texture", add a new texture, load the "sharp brush" texture from above and activate "rake" and "random" (image below).
The setup is now complete and you can start fixing the topology. Sculpt only with a small portion on the handle and the pommel.
When switching to Edit Mode, your topology should look more or less like this:
Back in Sculpt Mode, switch from "Relative Detail" to "Constant Detail", change the Resolution by using the "Sample" button to sample it from the newly sculpted portion of the mesh.
Next, hit "Detail Flood Fill" and then "Optimize" (image below).
Switch back from "Constant Detail" to "Relative Detail", change the Detail to 8.00, then select the "Smooth Brush" and select the "sharp brush" texture from above.
Turn on Matcap and select preferably a glossy one to highlight distortions.
Turn on "Rake" and "Random", then start smoothing the whole mesh. Be careful around the sharp edges, turn down the brush strenght to 0.2, if necessary.
Work your way around the mesh until everything looks smooth (image below).
Your mesh shold now be fixed and you can start to add the details. You might want to play with the Detail settings under Dynamic Topology to get the detail level you want to see in your result.