Looking at the XQuery code a year later, I do see that there are some things that are not necessarily written well. Try this one out:

select CAST(

REPLACE(

REPLACE(XEventData.XEvent.value('(data/value)[1]', 'varchar(max)'),

'<victim-list>', '<deadlock><victim-list>'),

'<process-list>','</victim-list><process-list>')

as xml) as DeadlockGraph

FROM

(select CAST(target_data as xml) as TargetData

from sys.dm_xe_session_targets st

join sys.dm_xe_sessions s on s.address = st.event_session_address

where name = 'system_health') AS Data

CROSS APPLY TargetData.nodes ('RingBufferTarget/event[@name="xml_deadlock_report"]') AS XEventData (XEvent)

If that doesn't quite get it, try pulling the target_data into a xml variable and then working off of that:

declare @xml xml

select @xml = target_data

from sys.dm_xe_session_targets

join sys.dm_xe_sessions on event_session_address = address

where name = 'system_health'

select CAST(

REPLACE(

REPLACE(XEventData.XEvent.value('(data/value)[1]', 'varchar(max)'),

'<victim-list>', '<deadlock><victim-list>'),

'<process-list>','</victim-list><process-list>')

as xml) as DeadlockGraph

FROM

(select @xml as TargetData) AS Data

CROSS APPLY TargetData.nodes ('RingBufferTarget/event[@name="xml_deadlock_report"]') AS XEventData (XEvent)