I found it weird how it was stated that Fairies are good at Illusion Magic when we didn't really see King or Elaine use it but with what happened in the last chapter when Mera tried to kill him, he might be able to use it.
I always thought it was their human disguises that counted as Illusion, since it takes a sustained spell and concentration(King complains of muscle soreness after releasing his fat nobleman form during his formal introduction to Elizabeth) to achieve and hold it.
The fact that Fairies can change their appearance, whether by just becoming fatter or ''becoming'' human is a hint at how it could be used in battle.
Or just blending in with the humans, like Helbram taking over that knight's form and infiltrating Hendricksen's HKs.
Does that mean that using the "Plague mode", Tristan will be able to also inflict illnesses and curses in the same way Percival was able to mummify living bodies in his "Death mode"?
I partly agree about inflicting illnesses and curses, but I think what he would do should be different from what Death Mode Percy does. I'm still holding onto that mode representing natural decay over Time. He can fastforward living beings' biological processes and kill them so...it would make Percy more similar to King using Fossilisation or Merlin's age-up enchantment. Tristan might end up decomposing all matter Arthur reshaped/purging and undoing all of Arthur's "creative" efforts

Ofc, only his share of the lot.
Also, if that's how their innate apocalyptic nature powers work then I'm truly worried about Gawain.... eating and regurgitating? Eww. XD
Gawain in chibi form before getting serious: "Hehehe, I am become Famine, Devourer of Worlds". On a more serious note, if Thetis' prediction based on Varghese's reports is true, they could try and realistically besiege Camelot(maybe once they've powered up and eliminated most of Arthur's henchmen), maybe Gawain could use her Sunshine in such a way to ruin all their food provisions. Apparently Arthur can't create new life in that dimensional pocket, they need to steal resources from Britannia.
This and the fact that he is the Knight of Plague could mean something.
I keep thinking he'll learn how to deconstruct the matter Arthur took away from Britannia and made it into New Camelot, while bridging the purging aspects of his Ark heritage with the consuming Hellblaze demons can summon/cast.
The plagues they are supposed to unleash will probably make sense in there.
I keep forgetting that we're only supposed to look at them in that nefarious KotA light from Arthur and Camelot's perspective...but they'll likely have to build up their strengths and skills even before getting in there. Arthur's declared War, so he'll likely intensify his meddling with Britannia.
Idk why he just doesn't activate his goddess healing then
Because for some reason he can't reconcile these powers and aspects of himself(he's not yet embraced the fact that he could protect people with his demon powers). Remember that Ark in many applications neutralises/purges PoD and lifeforms naturally containing it, or its excess, but it would still be harmful/painful. We don't even know how long it took for Monspeet and Derrieri to recover from Hikari Are after their Indura stint.
Now, regarding the subject of PoD making a demon "evil", I think the more accurate term would be "primal". Meliodas lost the ability to speak at Vaizel 1, he only recognised Elizabeth, and that by smell, and all the shadows and disembodied impressions the powerful demons give off are animal-like and animalistic, all the way to Indura mode. In AM, he also lost the moral filter. Or most of it.
Tristan himself seems to be able to preserve his sanity and a level of moral awareness, but he clearly experiences dissonance between his Id and Super-Ego, that means his Ego is neutralised while in demon mode.
He's aware of what he's doing, he's aware that some things would not be agreable to him normally, but his Ego is not there to help him control his physical actions.
Hmm, so Id=Demon Mode
Super-Ego=Goddess Mode
Ego=Nephilim/Pestilence...I guess.