Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
If you are tanking vs an AD team this is the build you want. Morde's job is to keep enemies in the fight when you want them to (hence the extensive speed boosts, ghost, and then many slows), and still have enough teeth to both keep them away from squishies and bring them down. Late Game very effective and the parts build well. Good for farming, early kills lots of health. No magic resist but a lot of health so don't use against AP teams.
Against an AP team this build still retains most of its health and movement speed, gains additional health regen, but sacrifices armor, A lot of resistance to AD and a good deal of AD for yourself. Still have a bunch of slows and AP heroes do not enjoy those so while your slows may be slightly less effective (no Omen), and you may be slightly slower with less AD, the AP heroes you are hunting are slower and have less armor themselves. Some may have a decent amount of health but few have any armor or abilites to run away when slowed.