sourcetype = evtx_from_UNC
index = main
The problem may be that you did not set sourcetype. Also, setting host to the string HOST seems silly and it would be better for you to know the host of the machine that had the file so I removed that. Note that in the end, the stanza header should have 3 total slashes (2 forward and 1 back) after the colon like this: [monitor://\var\log/evtx\*.log] or whatever.