In the last stanza, Thomas shows that "death shall have no dominion" - the ultimate power to destroy or end life does not belong to death.
A person may cease to be able to hear or see or sense. A person may endure all the hardships described in the previous stanzas. A person's physical body may decay, so that it no longer can be described as being a separate unit.
Regardless of what happens or how it takes place, Thomas is asserting that life will continue. Where a person may have once suffered, nature will find a way for life to arise again; "Where blew a flower may a flower no more Lift its head to the blows of the rain;"
The forces of the natural world will overpower death and, in the end, death will not be the final force.