Unaccusative Verbs

Among verb types, unaccusative verbs are intransitive verbs whose subject is the patient of the action. For example:

• nan'i-do wo ageru
  難易度を上げる
  To raise the difficult-level.
  • nan'i-do wo - patient, accusative case.
• nan'i-do ga agaru
  難易度が上がる
  The difficult-level rises.
  
  • nan'i-do ga - patient, nominative case.

Above, agaru is an unaccusative verb, and ageru is its lexical causative verb counterpart. Together, they form an ergative verb pair. The article about the verb pairs contains details on how both types of verbs work in Japanese.

By the way, the opposite of an unaccusative verb is called an unergative verb, whose subject is the agent, unless in passive voice.