In the City Communal Grounds tree on the building browser, above Town Center - Market Stalls are Slave Trader and Emancipation Ground. The latter is a faction building for the Langobards, whose description is:

The mythical origin of the Langobards is built upon the mantra "it is better to maintain liberty by arms than to stain it by the payment of tribute" - essentially, better to die free than live as a slave. The historian Paul the Deacon states that the Langobardi extended this right of freedom to those they captured along the way - freeing both prisoners of war and the slaves of others and incorporating them into their armies to increase their strength. This practice made the Langobards not only more progressive than their contemporaries, but also a composite people whose way of life was changed by this multicultural influence.
But there is no matching tent in the migration tree. Seems like an oversight, one does not need a city to set people free and hire them into your armies. Hopefully this is as easy as copying some tables. Only one building/tent would be needed because the tier 2 version in cities requires a resource, which are inaccessible to hordes.

I suppose I could sumbit this as a bug report to CA too, but I bet a mod would be made quicker. Normally I throw mod ideas at this wall to see if any stick and get made, but I hope this one will just because I suspect it would be easy.