Summary

TL;DR;

• Equip a spell to your right hand; any spell!
• Use the "Assign (Melee | Ranged) Historical Pact" ability
• In the menu, choose a Historical Pact slot (1 through 5)
• Find the Historical Pact power to which you assigned your spell (by number, and also the spell name will be in the power's name, such as "Melee Historical Pact 1: Firebolt" if you had chosen slot 1 and the "Firebolt" spell)
• Use the Historical Pact minor power for the desired spell; nothing special will happen until you activate the pact by doing this!
• Confirm that your Active Effects menu lists the pact as "channeled"; you should see something like "Firebolt is now channeled"
• Go hit stuff with your weapon and watch the spells fly!
  

As of 1.1.0, this mod requires a patch to add the pact-variant of each spell/magic-effect.  See the install/patch instrcutions at Install/Patch Instructions
This must be done before Pact Spells will work. If your spells and active-effects related to Pact-Weapon-Extended are not showing their names properly, you most likely forgot to do this step.

Please note that script-driven spells are quite funky to deal with, so they may not play nice, especially if they are concentration spells.  For example, FTweak's "Blood Bond" (concentration) does not damage the caster appropriately, where as the touch-version ("Blood Embolism") works just fine.  "Normal" concentration spells such as Flames and Sparks seem to work just fine, as they're not leaning on scripts.

If you use Requiem, especially 3Tweaks/FTweaks: ensure you have Scrambled Bugs (https://www.nexusmods.com/skyrimspecialedition/mods/43532) installed and set “Apply Spell Perk Entry Points: Multiple Spells” to true.

Longer Version:
This mod grants the player two abilities, hereafter know as 'Pacts': one for melee weapons, and one for ranged weapons.  What is a Pact?  Simply put, it's the linking of a spell to the player's attacks.  When a Pact is active (aka "channeled"), any subsequent attack that manages to hit an enemy will also attempt to cast the selected spell's effects as an on-touch/enchantment effect, as long as the player would otherwise be able to cast the associated spell (by having sufficient magicka, etc.).  The casting of a Pact will consume the same magica as though the player had cast it normally, the player will receive any blame/bounty they would have through normal casting.  Pact-cast spells are subject to the same perks or ability boons/detriments as the normal casting is, with the exception that Pact-cast spells cannot be empowered in the same way that normal casting methods allow. [This is an engine limitation, sorry]

This mod is built to avoid most common sources of mod-conflicts. It does not edit any existing spell, perk, book, etc. Only new content is added. A new game is not required, and is entirely save-game compatable. It is, however, built on scripts, so removing it in the middle of the playthrough should be done with thought. You can simply stop channeling a spell, and will never suffer any ill-effect or script lag. The scripts are purely event driven, and only fire when you use the powers, or attack with a spell channelled. No timers, cloaks, or other bloaty script behavior is used.

Gameplay

Each Pact is bound to the player, not the weapon, so if you switch weapons, the newly equipped weapon will also channel the spell without needing to reapply the Pact.  The Pact will apply for any weapon, including any melee, bow, crossbow, any weapon added by a mod, or whatever crazy thing some mod creates...anything, as long as it is a "WEAPON" record.

Further, any spell can be used in a Pact.  Even Mod added? You bet!  Even crazy things such as Ritual spells? At this moment, yes.  Using ritual spells will be absurdly over-powered/"cheese", but I'm leaving it to the user to self-regulate, for now.

Remember, also, that you have two Pacts: one for melee, and one for ranged, so while the Pact will apply to whatever weapon you equip, it is constrained by the weapon type (melee vs ranged).  The Pacts are not mutually exclusive (so yes, you may have both active at the same time).  You may choose to have both pacts associated to the same spell, or each have their own spell, and you may change the spell for each Pact at any time.

The player is also granted a series of minor powers (hereafter, "Historical Pacts"), whose purpose is to connect spells a player has chosen to a Pact (melee or ranged).  There are a total of 10 of these powers, five (5) for each pact.  They are numbered for easy identification (eg "Melee Historical Pact 1"), but their order/specific number is otherwise irrelevant.  Casting one of these Historical Pacts is how a Pact is activated/"Channeled" (this is step 4 in the "tl;dr;" section, above).  These powers can be hotkeyed to facilitate rapidly switching between spells; casting a Historical Pact after having already cast a Historical Pact of the same type will simply replace the previous spell with the new choice. There is no cost for casting any Historical Pact.  This is simply the means for choosing which Historical Pact you want active for future weapon-hits. Note: The memory-pact spells are added to the player after the first spell is assigned to the given slot; I did this to mimize the clutter for those that don't use all 5 slots.

Additionally, the player has two more minor powers (one for melee and one for ranged), whose purpose is to let the player assign the spell that is currently equipped in their right hand as the spell for which they'd like to use in pacts.  They are conveniently named "Assign (Melee or Ranged) Historical Pact".  Using the power will prompt the user to select which of the five (5) "historical pacts", belonging to the apprioprate weapon-type (melee vs range), will receive the current spell. This is step 2 in the "tl;dr;" section, above.  Once the user has made a selection, the affected historical pact is updated and you will see this reflected in your spell menu (under the powers/voice spells).  This does not change which Pact(s) are currently channeled.  It only changes what spell will be channeled the next time the Historical Pact is cast.  Further, using these powers to assign Historical Perks has no cost, as they're simply allowing you to set up the Historical Pacts as you would like.

Spells will not be cast unless a Pact is Channeled. To channel the pact, cast the "Historical Pact" power associated with the desired spell.  You will know what spells/Pacts are active by looking in your Active Effects menu.  You will know what spell is associated with a Historical Pact by looking at its name.  If a Historical Pact does not have a spell name within it's name, then no spell has been assigned to this Historical Pact, and channeling it will not allow a spell to be channeled.

Finally, you have two last minor powers: "Dispell Melee Pact" and "Dispell Ranged Pact".  Casting these does not change your Historical Pacts, but simply turns off the Pact abilities for the appropriate weapon type.  This is necessary for brawls, and similar events such as the sparring to join the Companions, as all of these events specifically prohibit the use of magic.  I didn't do anything to add this restriction, and I can't bypass it.  You simply need to turn off the pacts for these events.

GASP! This must be OP!
In general, no; however it certainly can be.  Using things such as ritual spells, or spells which were designed around specific constraints like long casting times or needing both hands, will clearly cause some imbalance.  How strong is that imbalance?  I can't say.  Should you care?  That's totally up to you.  Not enforcing these restrictions also means there won't be mod-conflicts because some mod works differently than others.

But I can use this, even with enchanted weapons!
Yep, sure can.  That was by design.  This is another point of concern I leave in control of the player.  You can disable the active Pact during those time you're using an enchanted weapon, if so desired; but I didn't feel it was my place as the designer to say you can't use Pacts wtih enchanted weapons.

What about the previous versions, "Requiem - Pact Weapon" and "Requiem - Pact Weapon Redone"?  Why should I use this one?
Simply put, this one is much better designed.  It supports any spell, mod-added, vanilla, whatever.  For reference, the previous versions had fewer than six (6) spells that could be pact-cast.  This was a huge shift in design, but actually ends up being logically simpler, with a huge bonus of removing 99% of the need for patching that the previous versions had.  Also, it lets you have separate Pacts for ranged vs melee, which doesn't sound big until you start playing with it more and find neat strategies of mixing spells/Pacts with enemy positioning.

Finally, because of it's underlying design, I can do cool things in the future with Pact-cast spells without touching the original spells, so it's sure to not interfere/conflict for npc's, etc.

Compatability

In short: very compatable.  There is absolutely nothing changed; only added.  Even for mod-added content.
However: I do use "onCombatHit" perk entry point, which is known to sometimes cause issues. The priority is low on my stuff, so worst case is my stuff won't fire, but other mods' affects will instead.  Check out "Scrambled Bugs" (https://www.nexusmods.com/skyrimspecialedition/mods/43532) to help avoid this issue.

Can I use {mod here}?
In short: yes; even overhauls like Requiem, etc are safe to use (I know, because I use Requiem with 3Tweaks)

Load Order/Dependencies

For Requiem users, place this before Requiem.  It shouldn't really matter, but there's also no reason to place it after, and placing this before Requiem is the general "safe" option.  Also, this adds spells and such, so remember to run the reqtificator after adding this.

Why do you list "Spell Thief" as a dependence?
That mod provided some really useful resources, specifically around spell-copying, and that mod author deserves a ton of credit.  Go support their mod, endorse it, say "thanks", etc.  Seriously, their work made this mod possible.  Unless there's a problem...in which case, complain to me :)

FAQ

I play {some mod}; do I need a (separate) patch?
I am not aware of any reason you would need one

I don't like they way you're doing ____
Fair enough;  feel free to drop a comment and I'll consider it.  Sometimes, it's a simple matter of different playstyles/goals, in which case I likely won't change the mod.  But there are likely things I have not considered, and I don't fear feedback.  Worst I can say is, "nah".

Planned Future Work

• Add a way to upgrade/enhance the pacts, to provide a sense of progression (status: brainstorming)