Read Recent SQL Server Deadlocks from system_health

SELECT TOP 20
       x.event_data.value('(event/@timestamp)[1]', 'datetime2') AS event_time,
       x.event_data.query('(event/data[@name="xml_report"]/value/deadlock)[1]') AS deadlock_graph
FROM (
  SELECT CAST(target_data AS XML) AS target_data
  FROM sys.dm_xe_session_targets st
  JOIN sys.dm_xe_sessions s ON s.address = st.event_session_address
  WHERE s.name = 'system_health' AND st.target_name = 'ring_buffer'
) AS tgt
CROSS APPLY tgt.target_data.nodes('RingBufferTarget/event[@name="xml_deadlock_report"]') AS q(n)
CROSS APPLY (SELECT CAST(q.n.query('.') AS XML)) AS x(event_data)
ORDER BY event_time DESC;