Ny method to obtain the Hidden Weapon Spell other than ranking in the Bell Keepers

Some of this information will overlap with the previous answer, but as that one doesn't address your question in full and relies mostly on speculation, I'll try to fill those gaps.

• Confirmation on the respawn limit: There are lots of people out there currently debating the relative merits and demerits of the system, so I think the existence of the limit is pretty well established. So far, the consensus seems to be that the limit is approximately 15 deaths/spawns, but this won't be known for certain until the PC version comes out and can be hacked, or until someone actually sits down, does a playthrough, and records all of the data.

• Name of the item which can reset the respawn limit: As @kiilla mentions, this is the Bonfire Ascetic.

• Which enemies the item will cause to respawn: It will respawn all enemies in the bonfire's area, including bosses. (More accurately, it resets the respawn limit on all enemies in the area -- meaning that if you've killed one enemy seven out of fifteen times, it will once again have fifteen lives as well).

• Additional effects the item has outside of just respawning enemies: This thread explains it much better and more succinctly than I could. The most important thing to understand is that it is actually making the area one difficulty level higher than it's currently set to (e.g., if you're playing on NG, the area will be set to NG+). Relevant portion of the thread quoted below.

• Loot on dead bodies is respawned (but not loot in chests, those stay opened and empty)

• Doors stay open, and any keys you've collected remain effective

• If there are any NG+ enemy types in the area, they will spawn

• NG+ BP enemies and NG+ BP NPCs will spawn in the area (the BP NPCs will not respawn if killed unless another ascetic is burned)

• Enemies have more hp, deal more damage, and drop more souls on death

• If there is a boss within the bonfire's area, it will respawn

• If the boss drops any additional loot in NG+ or higher, it will drop that loot when killed in addition to the loot it normally drops (boss souls, etc)

• Bosses have more hp, deal more damage, and drop more souls on death

Note that the effects of the Bonfire Ascetic are permanent and will continue into future difficulties as well. This means that if you use one in NG (thus making an area into NG+ difficulty), the area will be in NG++ difficulty when you move to NG+.

The MagAdjust of this game is just the damage listed that corresponds to the type of spell being cast, i.e. magic for sorceries, dark for hexes, and so forth. You'll notice on staves that some of them have equal magic and dark damage and some of them are weighted more heavily toward one than the other. The same is true for chimes. That helps you know if the item is better for sorceries, hexes, miracles, or a combination of 2 or 3 of them.

As far as what actually goes into the damage itself, it is a combination of the base damage as mentioned above, your INT and/or FAI depending on which school you are casting, and of course any reinforcements (+1 to +10) you've done to your staff/chime. Additionally, if you infuse Faintstone/Darkstone/etc to your staff/chime, you can add even more damage to the type of spell that applies to the type of spell you are trying to cast. That means you can make a staff weighted toward sorceries even more toward sorceries or you can more evenly weight it toward sorceries and hexes.

As far as wearables go, there are pieces of armor/rings that:

• Increase casting speed
• Give extra attunement slots
• Raise your INT and/or FAI by 1-5 points
• Give you more castings at the cost of HP

I don't know of any items that straight up give you raw spell damage like the Oolacile crown did in Dark Souls 1, but the +INT items definitely make a difference.

Ultimately, they've made your catalyst operate a lot more like a "normal" weapon in Dark Souls II. Just look at the damage number for the type of spell you are trying to cast. Also, pay attention to how well they scale for a given stat just like you would for any weapon. If there is an S under the INT picture on the screen for scaling, it will eventually do more damage than a similar staff with an A for scaling even if it started with less base damage.