Much of the time I think of items as addons to a build, and focus on the race, class, and feat progression when planning. However, in both Westmarches and many standard canpaigns, it is not unreasonable to be able to buy, craft, or at least request some specific items, at least going up to Rare rarity. That got me thinking about trying to optimize a build around the item, rather than the other way around.
Out of the Abyss provides a unique and interesting Rare item, the Stonespeaker Crystal. It has 10 charges, regains ~7 per day, allows spending some charges to cast the three Speak With spells, and, most importantly, allows expending a number of charges equal to a divination spell's level to replace one consumed spell component. The following divination spells have consumed spell compoments:
- Divination (4) 25gp¹
- Fortune's Favor (2) 100gp
- Legend Lore (5) 250gp
- True Seeing (6) 25gp
¹uses two components with combined cost of 25gp, so the crystal can only replace one of them.
If you play with EGtW, the obvious use case for this is the Graviturgy or Chronurgy Wizard, getting Fortune's Favor (upcastable for more targets, letting you safely cover a party at least once per day with a 1-hour long Lucky d20 each). You also get Legend Lore (cast for free once per day to keep improving your results about the same thing) as well as True Seeing as a Wizard, and since TCoE expanded Wizard spells to include Divination, you don't even need Ritual Caster Cleric to get the full set. It's nice to have the crystal on a Wizard who normally doesn't get Speak with Animals or Speak with Plants too.
The most broken use of this item that I can think of is, conveniently, with Chronurgy's Arcane Abeyance on Fortune's Favor. Cast Fortune's Favor at 5th level on four members of the party, and then also save a bead with a 4th level casting to re-up three party members' Lucky dice mid-combat, increasing the likelihood they actually use them. Between that, Chronal Shift, Silvery Barbs, and Convergent Future, you might as well be the DM now.
Even without EGtW, a Divination Wizard can benefit greatly from the Stonespeaker Crystal thanks to Expert Divination, effectively reducing the total burned cost of Legend Lore to one fifth of a spell slot. Again, after enough castings, you at least know as much as the DM does, and with Portent and Silvery Barbs can control the outcome somewhat.
Did I miss any synergies worth exploring?
Reduce your mental load to increase your speed: limit yourself to fewer additional stat blocks per encounter (e.g. if there is a boss battle with lair actions, you should ideally have 1, at most 2 additional monster stat blocks on top of that boss). Running something like 4 stat blocks is sufficient cognitive load that there is no way for you to play your monsters optimally and still be fast.
Reduce your reliance on small dice rolls to increase your speed: preroll a table of damage for any given statblock's actions, so that you can dole it out quickly (still roll the to-hits live tho, in case of crits and misses and reactions, and have misses consume a line from the preroll table too).
Add a low-value off-turn minigame for players to play: I like to make knowledge checks free (History, Nature, Insight, etc). You could allow players the chance to each get 1 free knowledge check in per round, but only off turn. That way if it's another player's turn, and they are thinking for a while, you can have the other players roll their knowledge checks and tell you what they are looking for. Resolve them later, at the beginning of those PCs' turns, which will also keep other players engaged since they are learning new info. Don't let them ask tho, save it for when you want to provide the distraction.