When Darius was born he promised Diana that he would raise Darius like would he his own son - and that's exactly what Nicolas dedicated himself on doing. He loved Fitzwilliam and adored Elizabeth however, there was just something different about Darius. Nicolas was very careful not to show his apparent favoritism though. However, as careful as a parent might be, a child always senses when something is amiss. Will caught on pretty quickly, Eliza decided it was because Darius was Nicolas' real son. In any case the siblings managed to gracefully accept the fact and never said a word about it against Nicolas. A gesture he was both most grateful and most ashamed for. Nicolas meant to treat Will, and Eliza and Darius all the same, but Nicolas just couldn't help but give Darius that extra cookie, or let Darius enter his 'sacred' office and draw while he worked, he certainly couldn't help the smile on his face every time Darius walked into a room. The fact of the matter was Eliza was right, Darius was Nicolas' son.
On the Night Nicolas got a call from a distraught Diana, he immediately apparated into her and Darien's home. What he was when he came did dismay him. The house looked like it was in the way of a small hurricane. Drawers were opened, it's contents sprawled on the floor, the furniture were rearranged and Diana was curled up on the floor, crying her heart out.
She told him heartwretchingly that Darien had left her. She showed him with a shaking hand a note that he must have penned:
"I'm sorry but you and the kids are safer this way. I don't want what happened to Calista and Armando to happen to us. I love you always, Darien." Nicolas remembered how absolutely pale she was when she informed him that she was in fact two months pregnant. He remembered how beautifully sad her blue eyes looked when she held on to him fearfully saying, "Nicolas, I'm afraid. Please don't leave me."
Nicolas stayed with her that night promising her that everything was going to be okay. He loyally held her as she cried herself to sleep. He vowed to himself that from that moment on he was going to make sure that Diana and the baby were well taken care of.
When Diana's Grandfather suggested that he marry Diana, he didn't really object to it as much as he would have if Diana hadn't been pregnant. He already vowed to himself that he'd take care of the baby and marrying her certainly made it much easier, not to mention authentic. He didn't care that this was actually his old rival's baby, the baby wasn't his anymore, the idiot left.
Nicolas was there when Darius first started kicking. Nicolas softly talked to Darius when he was still in Diana's womb. He tediously made Darius' nursery, without magic and refusing any help from Diana or Helena. He bought Darius all the latest in baby merchandise. He started to plan for Darius' future, much to the amusement of Helena. He started baby proofing the house, to Diana's annoyance. Dreams of what Darius would do when he was born filled his nights. When Darius would say first laugh, when Darius would say his first word, what his first word was? When Darius started walking, when Darius started talking. When Darius would fly his first broomstick, when Darius would get his Hogwarts letter. What house Darius would be in. He was in all senses of the phrase, an excited father.
Therefore, when Darius was born, he didn't need to promise Diana that he'd take care of Darius as his own son, because in his mind, it was already settled. Darius was his.
~~~
Darius was very upset when Will left for Hogwarts. He was so upset that he cried himself to sleep in Nicolas' arms. Nicolas carried him to his bed and carefully tucked him in. He kissed him goodnight and turned the light off. He was about to leave when suddenly Darius spoke. "Daddy wait." Nicolas stopped in his tracks and turned around to see beautiful blue eyes looking at him fearfully. "I'm afraid, please don't leave me."
Nicolas smiled obligingly and sat on the armchair by Darius' bed. "I'll be right here, son."