So dark knight will parry enemy attacks with their sword, will it also be dealing magic damage using its HP?
Y: When you activate a technique that the development team has been calling the "dark buff," MP will be consumed, and you'll be surrounded by a dark aura. A dark knight should always be fighting with this effect active. It's also possible to self-buff to increase damage by using the dark aura.
Will it also use magic?
Y: For attacking enemies, there will be both single target and AoE (ranged) magic available. Many people think that MP will be considered important to dark knights, but it will be more important to manage the dark power. However, the source of that power is MP, so in that way, MP is important. By the way, within the dark force, there is a system on another level which will further strengthen actions situationally. Tanking with dark knight is not like anything that we've had before.
With MP being important, the value of piety materia might grow in the future?
Y: Well, I guess there's some possibility it might for the people who are concerned about maximum MP on dark knight.
Are there also abilities which consume HP instead of MP?
Y: We were trying to decide for some time whether we should use HP instead of MP as the source of power for dark knight. We thought that by depleting your HP to enhance your defenses, such as Cecil in FFIV, it would be quite tricky in raids, so we decided to not adopt the idea of utilizing HP. We're still considering what support skills to implement in the future, so adjustments are ongoing.