As per Can the arcane/druidic focus staff double as quarterstaff?, I understand that I can use an arcane focus staff as a quarterstaff. I was wondering then if there was any limitation on that same staff then also being a warlock's pact weapon?
As far as I can tell there's no reason it shouldn't be allowed by anything in the PHB / DM guide / etc.
The Warlock class feature, "Pact of the Blade" gives you two options to make a pact weapon:
If you create a pact weapon, you can choose it's form, but are limited to melee weapons listed on the "Weapons" table in the player's handbook:
The warlock’s Pact of the Blade feature (PH, 107–8) lets you create a melee weapon out of nothing. Whenever you do so, you determine the weapon’s form, choosing from the melee weapon options in the Weapons table in the Player’s Handbook (p. 149).
So, you could select quarterstaff. But just because an arcane or druidic focus staff can act as a quarterstaff, that doesn't necessarily mean that a quarterstaff can act as a focus staff. Neither the arcane staff nor druidic staff are ordinary quarterstaffs.
Arcane staff, PHB p. 151:
a specially constructed staff ... designed to channel the power of arcane spells.
Druidic staff, PHB p. 151:
... a staff drawn whole out of a living tree ...
A created pact weapon certainly isn't drawn whole out of a living tree, and nothing in the rules indicates that it is designed to channel the power of arcane spells.
While, I am not aware of any magic weapon that explicitly acts as a arcane or druidic focus, Jeremy Crawford has given his opinion on twitter, that any magic wand can be used as an arcane focus. So your DM may take that into consideration, and also agree that any magic staff can also act as an arcane or druidic focus.
Also, there is nothing preventing your DM from putting such an item in your game, and per the rules you would be able to make it your pact weapon. As far as I can tell, this is your only option.
I don't see anything to prevent that.
In my world, I would rule that its function as a spell focus does depend on its specific form, so for it to function as such, you would have to summon it as a staff every time, instead of using the warlock pact weapon ability to reshape your (non-magical) pact weapon as you choose. There will be no arcane/druidic focus greatswords. i.e. it would be treated for this purpose as a pact magic weapon -- a real item you bond to, and you dismiss and re-summon in its one real form -- rather than a non-magic pact weapon that you create from thin air each time in the form you choose.
Pact of the blade says you can pick any melee weapon when you create it ( http://dnd.wizards.com/articles/features/rules-answers-april-2016) Arcane focus and druidic focus are not on the weapon table so they can't be chosen. You also can't pick improvised weapon for the same reason (so no stools).
