Mortick’s is, sadly, very outdated–the item was originally meant to drop in Patch 2.2.0. In 2.6.6, the item will have a few uses in 2 builds: IK HOTA and Zodiac WW.
In IK HOTA, it can replace Aquila Cuirass in the Cube. It will provide the same DR and better healing, but it won’t provide additional DPS. That means for most players, it makes the build slightly easier to play when you’re low-Paragon and don’t have many Augments. As you push into higher and higher tiers, however, you’ll likely trade Mortick’s for Magefist for the extra Fire damage.
Mortick’s functions much the same way for Zodiac WW. It will provide some extra defense for speed-farming T16 and will be a good substitute for Parthans as you farm lower GRs. However, once you start to push higher GRs, the DR offered by Mortick’s can’t compare to Parthans, and you’ll swap back. If there was a way to trade PoC + IP for Mortick’s, that would be a fair trade, but there isn’t.
Sadly, Mortick’s is not an end-game item in any build.
For more info, see Part 2 of this post: