Tiamat's Divinity was called True Death.
Just as Elena chased the peak of Life, Tiamat chased the absolute peak of Death. She took the concept past its regular limits and gave it more meaning, fleshing out its every aspect.
True Death was a concept that mirrored Nonexistence. It did not follow the cycle of samsara, but trumped life in all aspects, being the absolute end that consumed life no matter what.
The soul and ego did not get reborn, they were wiped from existence and replaced.
It was a terrifying definition that made the concept impossible to escape or cope with. It was impossible to see as anything other than what it was.
However, she was also an ambitious person. She didn't plan to use Death so single-mindedly.
Just as True Death encompassed the negative and horror-inducing aspects, it also covered the positive ones.