How to assign a lord to a unit on the battlefield

[Daniel BC]

I want to put a lord to lead or merge a unit on the battlefield but I can't, can any lord lead any unit or does it have to be a specific type of lord and a specific type of unit?

[Gresh]

Lords are individual, single-entity units. You can't merge or embed them with other units, as they function separately.

The closest you can get is to use control groups so you can select both the lord and other units at the same time. To do that, you select the lord unit and whatever other units you want and press "G" or use the group button on the toolbar.

[Invisible]

Lords can be assigned to siege equipment. Once in a time I had teclis shove a battering ram all alone.

[Xaphnir]

No lord can be merged into any other unit

not sure what gave you the impression that could be done

[grognardgary]

High light the lord and the unit and right click on the target. That is about the best you can do. Really though you're better off Having your unit charge the enemy frontally and then flank or reat attack it with you Lord especially if your lord or his mount causes fear or terror.

[Sputnik]

As above, Lords and heroes can't merge with units and lead them that way.

But those with the Encourage attribute will improve the Leadership (morale) of all units within their aura, and a little way beyond (mouse-over your guy to see his aura's full-strength AOE as a blue disk.)

Basic Effects:
+4 Leadership within 70m
The moral buff drops 1 leadership every 10m outside 70m.
It's a passive constant effect

Caveats:
Some campaign abilities may increase the aura size
Morale buffs provided by overlapping auras do NOT stack
Certain high tier units in most factions can also Encourage.

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There's other similar faction-specific AEO abilities that lords can acquire:
e.g.
'Hold the Line' (Empire)
Effect Range: 40m
Effects: +5 Defence Melee Defence, +4 Leadership