VLOOKUP type forumla needed

I have an Excel spreadsheet with two tables. The first table contains a log of data, in which there is no unique key, the second table is a list of unique keys and the most recent entry from the first table.

Table 1 - Assignment History
Columns:-
Date
Serial No.       - Device Serial Number, can appear multiple times.
Device Type   - Phone, Tablet, or Other
Action              - Assign or Unassign
Administrator - Name
Server               - Server A or Server B

Table 2 - Uniqueu Device List
Columns:-
Serial No.       - Device Serial Number, can appear multiple times.
Device Type   - Phone, Tablet, or Other
Last Action Date     - Date of last action (Currently this is a formula -

=MAX(IF(Assignment_History[Serial No.]='Unique Device List'!A2,Assignment_History[Date]))

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Last Action              - Should be Assign or Unassign, looked up from Table 1, based on Serial No. and the Last Action Date.

My question is what excel formula can I use to lookup the Action from Assignment History and populate this in my Last Action Column in Table 2 using both Table 2's Serial No. and Last Action Date?