The Undertaker had been spending his money over his career for one thing and one thing only.
To keep his son alive!
20 years ago, The Undertaker has a son, which has been striken by a bolt of lightening at birth, nearly killing the child.
Mini Taker has been kept alive only because his Father got a job with the WWE (WWF at the time) by striking a deal with "The Million Dollar Man".
Everybody has a price.
Every paycheck The Undertaker recieves is given to his son's medical bills.
The Undertaker also reveals that he was "chosen" to be "The Undertaker" as he would be given immortal powers in order to help save his son.
His son finally wakes up from his lifelong coma near the end of The Dibiase Snr/Jr Vs Taker feud and attacks his father after Taker defeats the Dibiase's. The two Ted's then take responsibilty for "The Son of The Undertaker" saying they bought him out.
This leads up to a match at Wrestlemania 27 where The Undertaker is scheldueled to face his son.
The Undertaker's son attacks the Dibiase's The next night on Raw proving the Dibiase's never bought him out.
The Undertaker calls out to his son repeatedly while recieving no answer from his offspring.
At Wrestlemania 27, The Undertaker's son defeats his Father.
Afterwards, a casket is brought to the ring by druids. The Undertaker's son finally proclaims his first words
"It is time Father.."
The Undertaker is carried into the casket by the druids and The Undertaker's son becomes "The New Phenom" and does his Father's infamous pose as a way of respect for his fallen father.