Howdy all!

Being a first legion fan, and eagerly awaiting the arrival of Forcas, I was looking at his character art and noticed the Latin inscription on his sword.

It appears to be "Mortes Interficium".  Now, I am not language scholar, but this is my attempt at its translation.  First we approach with Latin.

Mors is the noun root word translating to "death", "annihilation", or "a corpse" derived from the singular Mortis.  Mortēs is the plural genitive of Mors, and can either be nominative or accusative.  In nominative cases, Mors would be the subject of a verb, so in the example of "Dark Angels hunt the Fallen," the Dark Angels are the nominative subject.  The Latin accusative case usually marks the direct object of a transitive verb, indicating the recipient of the action.  This suggests that the Mors is being acted upon.  Also note, that Latin genitive nouns followed by verbs usually indicate the object of the verb.  

Still with me so far?

Interficium is derived from the verb interficiō, which means to "slay", "kill", "destroy", or "assassinate".  Interficium is the first-person singular future active indicative form, which is to say something will happen, an example being "I will eat".

Put together, we end up with two literal translations.
●  In the nominative, "Death(s)/corpses will slay/destroy/kill" 
●  In the accusative,  "(I) will slay/destroy/kill death(s)/corpses"

Personally, I like "I will slay death" as a translation even if the translation is awkward because death in this case is in the singular.  

However, we are still not certain what the Inner Circle Companions are, and it has been suggested that the could be redeemed members of the Fallen chosen by the Lion.  This would put an interesting twist on the translation if you loosely interpret that the Fallen are in figuratively dead (corpses).  That would lead to a translation similar to "The Fallen will destroy/slay/annihilate".

Lastly, There could be a case made that it is another language, but I find this less likely given how James Workshop likes to used Latin in the Imperium.

If you have gotten this far, and know anyone who has actual Inner Circle Companion models, I would love to know what other inscriptions are on the Calibanite Greatswords.  They could aid in the translation.

TL;DR: The inscription on Forcas' sword is Latin for "I will slay death", or if you believe that the Inner Circle Companions are "risen" Fallen that have been redeemed, "The Fallen will slay".

Forcas' Sword Inscription