Conduit

A conduit is a block that gives nearby players the Conduit Power status effect and attacks hostile mobs when powered by placing a certain configuration of blocks.

Obtaining
Conduits will drop as an item when broken with any tool or by hand, but a pickaxe is the fastest way to break it.

Conduit power
When activated, conduits give the "Conduit Power" effect to all players in contact with water or rain, within a spherical range of 32-96 blocks. This effect stops the breath meter from decreasing, gives underwater night vision and increases mining speed. Conduits can be activated in any biome and at any height or depth.

To activate, a conduit needs to be surrounded on all sides by water (source blocks, flowing water, and/or waterlogged blocks) and the conduit and water must be enclosed in an activation frame. The frame is built up of blocks in three 5×5 open squares centered on the conduit, one around each axis. Only prismarine, dark prismarine, prismarine bricks and sea lantern blocks in the frame contribute to activation. A minimum of 16 blocks are required, and produce an effect radius of 32 blocks. Prismarine-type slabs (including double slabs) and stairs cannot be used to activate the conduit.

The effect radius of the conduit is 16 blocks for every seven blocks in the frame, though the effect does not activate until the minimum of 16 blocks are included in the build. Thus, it extends to 48 at 21 blocks, 64 at 28 blocks, 80 at 35 blocks, and 96 with a complete frame of 42 blocks. A complete frame also carries the additional advantage of attacking hostile mobs within 8 blocks of the conduit by every 2 seconds, if they are in contact with water or rain.

Other blocks used as part of the frame (including prismarine stairs, slabs, and double slabs) have no effect on activation, and can be omitted. Any blocks outside the frame but within the 5×5×5 cube that encompasses it likewise have no effect.

Light source
Conduits emit a light level of 15, the brightest possible light level in the game, whether activated or not.

Permanent water source
The water inside a conduit cannot be absorbed by a sponge, allowing it to act as a robust source of water.

Trivia

 * When it is activated, the conduit displays the heart of the sea's texture in the center of its model.
 * The eye of the conduit will show whether it's hunting for hostile mobs or not: it will show an open eye when it's looking out for hostile mobs, and a closed eye otherwise.
 * If it had been possible to activate a conduit with fewer than 16 blocks, its effect range would be 0 at 0-6 blocks, 16 at 7-13 blocks, and 32 already at 14-15 blocks.