Officially even Warlock is just here to deliver support in a way that Clerics have a better life. Guess this is how IMC wanted warlock, support to increase Cleric dmg to kill monster such that you don’t get any souls for absolutely 0 dmg. 
There are only a handful of skills in Tree of Savior that give characters the ability to change the property of enemies, and Mastema is one of them (another prime example is the Druid’s Chortasmata).
The main advantage of Mastema is that, as its stigma changes enemies to Devil type, characters with strong attacks against this property can use it to deal extra damage to any monster (this is especially useful to your Cleric friends). On the other hand, we thought 5 seconds was a little too short to have a significant impact in actual combat, so we decided to give a big boost to the duration of Mastema.
Our hope is that this will encourage Warlocks to use Mastema in a wider range of situations and in combination with their own as well as other characters’ skills.