In January 2019, Nicola Kilner, CEO and co-founder of the now billion-dollar skincare brand The Ordinary sent the hardest email of her life.

In the email, Kilner confirmed the sudden death of her then-business partner, Brandon Truaxe.

“I can’t believe I am typing these words. Brandon passed away over the weekend. Heartbroken doesn’t come close to how I, and how I know many of you, will be feeling,” read the email from Kilner.

“All offices, warehouses, factories and stores please close today and take the time to cry with sadness, smile at the good times we had, reflect on what his genius built and hug your loved ones that little harder. We are all in disbelief and shock but I will be in touch again very soon. I love you all incredibly much, as did he.”

Watch: There are 40 aliases of Samantha Azzopardi, making her an international con artist. Post continues after video.

Going on four years since the death, Kilner has shared intimate details surrounding that time and her tumultuous relationship with her co-founder in an episode of Diary of a CEO.

In the interview, Kilner speaks about building the beauty empire at just 24 years of age, alongside business partner and friend Truaxe.

How did Nicola Kilner and Brandon Truaxe meet?

Kilner says it all began with her job at Boots, a pharmaceutical giant in the UK, which was where she first noticed a price gap in the market.

In a phone call with Truaxe, Kilner says she "always wanted to do her own thing". But the two decided to join forces as Truaxe had just quit his job and wanted to build a multifaceted company, which would parent companies across a wide range of industries.

Truaxe decided to check in with Kilner, and the two decided to launch the company now known as DECIEM.

Kilner says this pairing was an easy decision, considering they had worked great together in the past and that their particular skill set complemented one another, which Kilner says was invaluable for starting their business.

They both agreed that developing a strong company culture was going to be an important focus for their new business venture.

Kilner says it's because of this that they built their company with a similar structure to that of a cult.

It was Truaxe that has the philosophy that to create a thriving business you need to be a family. The pair say they were able to do this due to the fact that they were building from the ground up.

"We were a startup, we couldn't afford the experience, everyone who was hired was pretty much straight out of university which meant no one really had preconditioned ideas about the beauty industry," says Kilner, which ultimately led the team to become "insanely close".

Soon they were working, travelling, eating, sleeping, partying, practically doing everything together, each andfor years on end. As you can imagine this created a deep emotional bond between Kilner, Truaxe and their dedicated employees.

As The Ordinary became increasingly popular for being the only 'abnormal beauty company' on the market – a phrase the company continues to use – things began to change. Kilner and Truaxe had just signed the company's biggest deal with The Estee Lauder group, merging their corporate powers and launching the company far beyond anything they could have ever imagined.

What were the first signs that Brandon Truaxe was acting strange?

What should have been a very exciting time for Kilner and Truaxe was very short-lived.

That summer break Kilner and her partner travelled to Australia to enjoy a well-earned holiday, while Truaxe was heading to Amsterdam to experiment with psychedelics.

Kilner still actually defends that Truaxe was not into illicit drugs by nature, rather she describes him as a "diet coke and fries with ketchup kind of guy." Unfortunately, it seemed that Truaxe soon found himself obsessing over the effects of experimenting with drugs hoping the "magic mushrooms might unlock certain parts of the brain", says Kilner.

Then Kilner received a strange call from Truaxe.

She describes the call as disturbing and recalls feeling that something was very wrong. She felt as if she had never met the man on the other end of the phone, despite practically being family.

Truaxe asked her if she had her passport on her and requested her to go straight to the airport, that she needed to come to Toronto, Canada immediately.