Literally:
* Descend: This is the most common opposite, meaning to move downward.
Figuratively:
* Decline: This means to decrease or become worse.
* Fall: This can refer to a decrease in something like status or power.
* Diminish: This implies a reduction in size, importance, or value.
Other possible opposites:
* Recedes: This means to move back or away.
* Degrade: This implies a lowering of quality or value.
The best opposite for "ascend" will depend on the specific meaning you are trying to convey.