"Windows of Opportunity" are the collective name for a number of artifacts that were gifted by gods to their leading disciples to enable better communication while these deities still stalked the Prime Material Plane. Theoretically they could still serve this purpose, but in modern times, they serve as powerful relics of divination and conjuring, that mirror the spell slots of the user, for certain spells. 

Major properties.

• Each day, an attuned spellcaster can use the Window to cast any prepared divination spell as a ritual without using a spell slot up to the equal of the slots the spellcaster has available. 
• The spellcaster can also cast three prepared conjuring spells as a ritual each day.

A ritual spells usual cast time is reduced by 90%, and a non-ritual spell has a cast time in minutes of 10 times the spell level where the user spends meditating within the Window. After the first ritual, repeated casting of the same spell that day is instantaneous ( eg, 3 lots of sending requires only the first 30 minutes meditation).

Use of the windows exposes the spellcaster to the attention of other divining creatures across the planes. For each use of the window on a day, the spellcaster must roll their spellcasting ability plus their modifier. The difficulty is 10 plus the level of the spell cast. Each successive spell that day adds to the difficulty.

A fail means the use of the window has been visible: It caused a ripple through the weave that could have been noticed by other creatures. 

A creature can only have observed the user through the mirror if:

• They beat the caster’s check with their passive or active spellcasting score of 10 plus their spellcasting ability modifier; or appropriate WIS/INT/CHA modifier if the creature is not a spellcaster. 
• They were reasonably expected to be engaged or connected to the divine or conjuring realms of the Weave at the time - crystal ball, window, or by their nature. This could be determined by percentage die or DM fiat, as suits. For example, a failed check by 5, or by 10 can be considered to have random minor or major contact. A creature could also have been actively searching or waiting for the spellcaster to make an active contested roll. This, and other factors such as relationship, proximity, and time - whether the spellcaster was made visible earlier that day, the previous day or week, etc, how long they are connecting (10-90 minutes) - can modify this percentage check. If observed, 

A watcher can observe what spells the user casts and gather all the information they do. This check must be refreshed for each spell that day, but they are otherwise found as long as the watcher stays connected to their own scrying device or trance. 

A watcher can be revealed if

• It makes a spell attempt
• It fails an observation check
• It is found by an active ability check by the user looking for watchers.

Two parties connected through the window in this manner can cast spells on each other in the normal manner, as if in range and normal cast time. Spells cast on an unwitting observed party have advantage. These include scrying, charms, feeblemind, and all reasonable effects - even physical attempts entangle.

 If combat ensues, initiative is rolled round by round via spellcasting ability, thereby only being an attacker and a defender. The party with initiative is the "attacker" - they can escape, or keep the contact and attempt to inflict an appropriate spell. 

Appearance and minor powers.
Each Window is different in appearance and has their own unique pedigree and powers. 
The Sea Tyrant’s Scale is also known as the Boon of Zeboim. It is just under a metre in length, and heavy, with a horned keratin shell braced by a low reptilian creature. The shell resembles the symbol of Zeboim. and is is hinged so it unfolds in two halves down the middle. Inside is a mercurial surface like a smooth silver dish, neither liquid nor solid nor gas, like a mirror the eyes sink into without reflection (this "window" is an effect shared by all these artifacts). The scale comes with a cloth of folding: a large black velvet cloth which when folded over the object allows it to be folded over and over down to slender wallet size.

Spells. The scale has 6 charges. While bearing it, you can use an action to expend 1 or more of its charges to cast one of the following spells (save DC 18) from it, if the caster can cast spells of those level: fog cloud (1 charge), shatter (2 charges), call lightning (3 charges), tidal wave (3 charges), maelstrom (4 charges). Additional charges raise the spell 1 level. The scale regains 1d6 expended charges daily at dawn.

While attuned, the user gains the following beneficial effects:

• Cannot be charmed or frightened; 
• Resistance to lightning and thunder;
• Both water breathing and water walking

While attuned, the user suffers the following detrimental effects:

• Casters of good or lawful alignment suffer 2d6 temporary reduction to their hitpoints and lose 1 HD for rest purposes each daily. This effect is removed after a long rest, but suffered anew each dawn. The effect stacks without a long rest, and for users who are both lawful and good.
• While attuned, users not aligned with the Tempest domain, The Devourer, Malar or Zeboim feel physically ill, parched and queasy as though their stomach was lined with seawater. The user suffers disadvantage on strength and constitution saving throws.

Notes: Disadvantage: Constitution Saving Throws, Disadvantage: Strength Saving Throws, Resistance: Lightning, Resistance: Thunder, Immunity: Charmed, Immunity: Frightened, Bonus: Hit Points, spellcaster