Conduit

A conduit is a beacon-like block that provides Conduit Power and attacks hostile mobs underwater.

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 rain or water, within a spherical range of 32-96 blocks. This effect stops the breath meter from decreasing, gives underwater night vision and increases underwater mining speed. Conduit power also kills any hostile mobs within range like drowned and guardians. Conduits can be activated in any biome and at any height or depth.

To activate, a conduit needs to be in the center of a 3×3×3 area of water (source blocks, flowing water, and/or waterlogged blocks), which itself must be enclosed within 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 range of 32 blocks. Prismarine-type slabs (including double slabs) and stairs cannot be used to activate the conduit.

The effect range 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. 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 light level in the game, whether activated or not.

Block entity
A conduit has a block entity associated with it that holds additional data about the block. The conduit's block entity ID is. It is not known what this block entity is used for.

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.