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Variety is the spice of life and all, but if you're going to do the shadowsword thing in a truly fluffy way you'll only use it in matches against Titans as part of a solar auxillia or cults/militia list.
The fluff on the shadowsword is pretty consistent about its titan-hunting battlefield role.

There is a mention of solar auxillia using them against traitor titan legions with huge losses and also as an artillery peice against standing structures (primus redoubt?) so a loyalist seige breaker specialist legion is probably as close to 'fluffy' as it's going to get i think.

It's worth remembering every legion has at least one of everything; and it could even have been found with a dead crew by your marines during an extended conflict (waste not want not etc)