Explanation of all canon versions;
Arella (her name was Angela) was a Christian, her father was very strict, her relationship with her family was bad, which led to her running away from home. She ended up joining the satanism group, they wanted to give a bride to a demon and Angela was chosen, they managed to summon the demon, it was Trigon who appeared to them. Thus Angela met Trigon for the first time, it has also been mentioned that Trigon appeared in human form, she wanted to understand this demon so she asked about his truth; Trigon calmly answered her:"I am everything that I am needed to be." Angela agreed to go with him, she was fully aware that he was a demon.
They made love, while they did this Trigon appeared to Angela in his true demonic form,Angela was surprised, {the reason was when someone, seeing the true devil's form, would be really amazed} At this moment, Trigon smiled with an evil smile; Angela knew that Trigon was planning a great thing. She argued with him. Trigon brought her back to Earth. It was not mentioned why he returned it to Earth,but later it was found that he said: “I took your mother gave her my seed, then returned her to earth lest my enemies slay her before you were born.” to Raven from {The new teen titans 1984}
Trigon sent Angela to Earth and their relationship became incomprehensible At least for Angela, but Trigon was waiting for her to give birth to his child, Angela meanwhile, was confused and frightened when she knew she was pregnant with a demon child. She chose to commit suicide; At that moment people from Azarath appeared to her, they took her with her until she gave birth to her daughter {Raven}, Angela became called “Arella”. She took care of her daughter; Arella loved her daughter more than anything.
Until Trigon appears on the Azarath, Arella meets Trigon again, and knows he wants to use their daughter as a tool, in order to enter and destroy the Earth, Arella tries to protect Raven but Trigon turns her back,refusing to kill her for saying;“ I still have several years, need of her". They met a over and over, it was strange that Trigon had never tried to kill her. Trigon turns her back her every time, and prove to her that she is a weak human and laugh with every arrogance.
So Here...!
Did Trigon deceived Arella in her appearance as a human? Answer “No” Trigon told Arella that he everything he needed to be,I think these are words that make sense to think that can be with every appearance he wants,his logical thinking has another original appearance. I think Arella understood this,though she ignored this,she accepted in Trigon as a partner.
Was Arella raped? I didn't see this happened, this makes no sense to me to say that Arella raped! rape is something that takes force, Arella was fully aware that the demon with whom she was! fully aware of what it meant to be accepted as a partner and a sexual relationship, she accepted to do this with a demon. Whatever his appearance he is A demon he didn't assure her of his appearance since told her that everything he needed to be. She was with Trigon of her own will and that's enough to say that she not raped.
One of the versions, Trigon recounted how he met Arella in his true demonic form
where he said; “Meet Arella with his true form, Arella accepted him and lived with him, she was pregnant with his child, Arella stayed with him until give birth to their daughter a few months later, Arella knew Trigon's purpose was Earth. A dispute has occurred between them! rebellious Arella didn't accept that their daughter be a tool, Arella doesn't want to harm her kind {the humans}, arrogant Trigon Can't imagine there was a woman opposing him, The conflict didn't end for good between them, it ended up that Arella took Raven and she quickly left Trigon {run away}. She managed to escape and ended up on Azarath.”
The relationship/ feelings between them:
It's hard to say that Trigon loved Arella, I talk about a character more than saying it is complicated, Trigon is an overbearing demon, he is bossy even he proud of his evil, he says about himself that he is the most powerful evil the universe knows. I was hopeless to find good feelings inside this demon! I think Trigon was interested in Arella, the moment he didn't think would hurt Arella. He only fought her when he realized that she wouldn't be with his side.
Arella was exceptional for him! Trigon does't admit to himself that Arella is exceptional for him, he is bossy, arrogant, He sees himself as the greatest demon in the universe and he can't admit it that he completes with something else. while Trigon said in {Teen titans 2011}, he wanted Arella to stay with him to raise Raven. he convinced himself that Arella must stay with him, In order to raise Raven and learn her best! this is what Trigon convinced himself. he is arrogant ,he can't admit that he is the greatest thing in the universe complete with something else, (This is the main idea, which Trigon get out of.)
So found himself a better way to express this. The absolute truth, Trigon believes that feelings are a weak thing, he does't want to be part of this. Trigon, despite his interest in Arella, has appeared toxic, but he rejects any feelings.
Arella's feelings for Trigon are tense and confused. Arella, she wanted the love, later became only wanting hope from him. As she cried in {The new Teen Titans 1980} she said: "I above all, know the full extent of Trigon's madness and only I have seen how truly evil he can be!." She was very sad for Trigon's actions, Arella was trying to talk to him every time, but she got to nothing.
The truth is unclear because of Trigon's arrogance; Trigon had feelings for Arella, she piqued Trigon's interest, Arella was a rebel which made Trigon want her more! This is an unclear fact which will always be dust due to Trigon's arrogance!