The release of the Blood of Luclin expansion also added a new persistent mercenary buffs. The following is from the dev team regarding these buffs:

Persistent Mercenary buffs

Mercenaries that have been unlocked to hire anywhere grant a persistent "Mercenary Battalion" buff even when suspended. Every mercenary that you have unlocked to hire anywhere may contribute to Mercenary Battalion, with higher tier and higher rank mercenaries contributing more. While each type of mercenary will grant a different buff, all mercenaries of that era contribute to the effect of all other mercenaries of that era. The last mercenary summoned will automatically cast its Mercenary Battalion buff on the owner whether it is active or suspended.

To make a mercenary contribute to the buff, summon a mercenary that you have unlocked to Hire Anywhere which is also at least rank 5, and its contribution will be added to the total. Contribution is increased at rank 5, 15, 20, and eventually at rank 25 when that is unlocked. While you have a mercenary summoned, you can right click on it to check how much it has contributed to the total.

Contribution can be increased up to 100% and will not further increase the effects once 100% has been reached.

Each mercenary grants a specific buff based off of its corresponding mercenary type. Further, more powerful/rare mercenaries provide a more powerful version of the effect, thus a Celestial mercenary will grant a more powerful version of the effect then a Treasured mercenary.

Current buffs are Shattered Lands, for all mercenaries released before the Blood of Luclin expansion.

• Mercenary Battalion: Shattered Lands (Archer)
• Mercenary Battalion: Shattered Lands (Caster)
• Mercenary Battalion: Shattered Lands (Healer)
• Mercenary Battalion: Shattered Lands (Melee)
• Mercenary Battalion: Shattered Lands (Support)
• Mercenary Battalion: Shattered Lands (Tank)

And Blood of Luclin for all mercenaries released since the expansion launched.

• Mercenary Battalion: Blood of Luclin (Archer)
• Mercenary Battalion: Blood of Luclin (Caster)
• Mercenary Battalion: Blood of Luclin (Healer)
• Mercenary Battalion: Blood of Luclin (Melee)
• Mercenary Battalion: Blood of Luclin (Support)
• Mercenary Battalion: Blood of Luclin (Tank)

Mercenary rarity, at the listed level, will grant the displayed contribution to that seasons Mercenary Battalion buff. This table is total, not additive.

So a rank 5 Celestial merc will give you 4 contribution. A rank 10 will give you 4 more, for a total of 8 (listed on the table as 8).

Additional Note From Mum:

Word is that if your hire anywhere merc is not granting a buff, the first thing you should try is to hire a different merc, then dismiss them, then summon the merc that is being stubborn about the buff. If that does not work, you may need to /bug that specific merc so that it can be looked at.