User Story is Done according to the agreed upon Acceptance CriteriA. Thus, the role of Acceptance Criteria in maintaining quality is critical and needs to be clearly understood by the team. It is the responsibility of the Scrum Master to ensure that the Acceptance Criteria for a User Story are not changed by the Product Owner in the middle of a Sprint. Partially completed User Stories are rejected as not Done and moved back into the Prioritized Product Backlog.
Reference:
http://blog.scrumstudy.com/scrumstudy-scrum-video-on-acceptance-criteria-and-prioritizedproduct- backlog/

Because stakeholders have an interest in the project, it is important to communicate with them regarding risks. Information provided to stakeholders related to risk should include potential impact and the plans for responding to each risk. This communication is on-going and should occur in parallel with the four sequential steps discussed thus far-risk identification, assessment, prioritization and mitigation. The Scrum Team may also discuss specific risks related to their Tasks with the Scrum Master during Daily Standup Meetings. The Product Owner is responsible for the prioritization of risks and for communicating the prioritized list to the Scrum Team. An important tool which can be used for communicating information related to risks is the Risk Burndown Chart.
Reference: http://blog.scrumstudy.com/risk-management-in-scrum/

The key inputs of creating user stories are Scrum Core Team, Prioritized Product Backlog, Done Criteria, and Personas. The most import tool is User Story Writing Expertise. And the major outputs are User Stories and User Story Acceptance CriteriA.