Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.