Background summary:
Large numbers of psykers, some of whom are chaotic and might summon daemons for backup, with time to prepare and perform rituals, are a tough enemy even for Space Marines. Therefore, I propose to take advantage of the fact that we have a serious airforce (see
Forces) and advanced technology, while the Djinncallers merely have some flying units, by bombarding them from the air. The first plan attempts to starve them out and take their stuff mostly intact by destroying their supplies, at the cost of risking upset Elves if we sink an Ulthuan ship. The second plan attempts to reduce our casualties by breaking their stuff before we land, at the cost of having less loot.
I encourage people to vote just the plan name if you like one of these, not copy the entire block, so I can make inevitable minor edits while keeping the spirit of the plans.
[X] Plan Starvation
-[X] Preparation phase
--[X] Announce and broadcast an embargo of the Sorcerers' Isles until they surrender. Any foreign ships docking there will be sunk for aiding the enemy.
--[X] Pilots: Fly over the islands, burn fields and bomb food storehouses, take pict-captures and recordings for further inspection. Bring Interceptor escort in case of Air Djinn.
--[X] Mechanicus: Assist with augur scans and analysis to locate concealed storehouses.
--[X] Librarians and whoever else works with warpstone: prepare warpstone-laced bolter rounds for use against daemons.
--[X] Militarum: Practice amphibious landings and artillery transport.
-[X] Offer terms of surrender: our Librarian will inspect the Djinncallers and execute any Chaos-corrupted ones. The rest will become vassals, subject to negotiation.
-[X] Invasion phase
--[X] Astartes: Partial decapitation strike at the capital of Water Island using the majority of the Chapter's forces, then moving to help secure a port for the Militarum.
--[X] Interrogate captives, eat brains, destroy control items, or otherwise make strait-guarding Djinn stand down where possible.
--[X] Militarum: go by ships to Water Island, set up for cautious advance and artillery support once they land.
--[X] Continue rest of invasion with Militarum leading, Astartes held in reserve in case of enemy champions or other surprises.
Video recordings are an
awesome tool for ferreting out depots. Once you observe a covered cart turning out to be food, you can play the tape backwards and see where that specific cart came from, rather than trying to remember every cart's route and mentally sort out the decoys.
I'm planning the invasion on the assumption that they won't surrender, but I think the condition I've given is fairly clever and should appeal to both sides. (Assume that other reasonable surrender conditions about vassal states apply.)
The Astartes will be expecting a lot of purges of untrained psykers haphazardly grasping at the Warp and getting corrupted, and the survivors can be kept at arm's length and potentially used as an expendable force against other Mallus Psyker Bullshit.
The Djinncallers will be expecting a lot fewer purges (and OOC, I expect they'll be correct) because Araby is so far from the Polar Gates, and the influence of Chaos and magic generally is weaker here, which is why they can reliably bind Djinn, something that would lead to a lot of free elementals running amok if wizards in Sigmar's Empire tried it.
Alternatively, the simpler approach:
[X] Plan Devastation
-[X] Preparation phase
--[X] Pilots: Fly over the islands, repeatedly bomb anything that looks sorcerous or towerous or important, except the Fire Island capital. Bring Interceptor escort in case of Air Djinn.
--[X] Librarians and whoever else works with warpstone: prepare warpstone-laced bolter rounds for use against daemons.
--[X] Militarum: Practice amphibious landings and artillery transport.
-[X] Invasion phase
--[X] Astartes: Decapitation strike at the capital of Fire Island using the full Chapter's forces.
--[X] Militarum: go by ships to an outer coast while the Djinncallers are in disarray and likely concentrated in the capital.
The idea here is to have them think the capital is well protected and safe during preparation, enhancing the effect of our decapitation strike, so our Astartes can shoot as many of their Djinncallers as possible while they struggle to figure out what the hell is going on. This kills most of their mages and their magical tradition, and probably burns a lot of heretical tomes in the process, but we still get some artifacts since Astartes aren't city-destroyers the way future bombers are.
I
might edit it to add a single bombardment of the capital at the start of the invasion phase to kill a few more of them before the decapitation strike.
Do we know enough about them that it's worth trying for shouts of "The Caliph has betrayed us!" or "The [faction] has gone over to the invaders!" in Arabic (Arabyese?) during the attack so they'll be even more confused? Or is that perhaps a degree of micromanagement I should just leave to the Astartes?
@FractiousDay these will probably need some adjustment to fit your expectations of a plan, but I trust the general idea is clear.