What consequences are there for use of chemical weapons under the CWC and related treaties?
The convention is here. Under Article X para 8, if chemical weapons have been used against a State Party, it may request assistance and may receive assistance and protection against such an attack (the request goes to the Director General of the Technical Secretariat of the Organisation for the Prohibition of Chemical Weapons, who transmits it to the Executive Council and all State Parties). There shall be timely investigations and reports. If "immediate action is indispensable, the Director-General shall notify all States Parties and shall take emergency measures of assistance, using the resources the Conference has placed at his disposal for such contingencies".
Article XII addresses measures to assure compliance, if a State Party has been requested to conform but fails to, the Conference may "restrict or suspend the State Party's rights and privileges under this Convention", they may "recommend collective measures to States Parties in conformity with international law", "in cases of particular gravity, bring the issue, including relevant information and conclusions, to the attention of the United Nations General Assembly and the United Nations Security Council". The UN, then, is empowered to discuss the matter at length, and perhaps issue a resolution. For example, the UN passed Resolution 2118 requiring Syria to destroy its stockpile of chemical weapons, which the OPCW reported is "largely on schedule". As part of their enforcement actions, inspectors were sent to Syria.
A further possible consequence is that some state may take military action against offenders. While this is not explicitly a consequence of any convention, it is a politically-possible outcome.