You place the crystal down, it then forms a floating construct that is small in size and sheds dim light in a 5ft radius around it. When the casting the spell you choose one damage type that isn't bludgeoning or slashing.
The construct has an amount of Hp equal to five times the spell level and an AC equal to your your spell save DC. The construct is immune to psychic and poison damage. It's speed is zero. The construct always goes last in the initiative order. (more stats that will be shown in a separate stat block)
On its turn it fires one beam at a creature you choose within 60ft of it dealing 2d8 damage of the damage type you chose when you cats the spell. In addition, when you cast the spell you can choose to modify it in one of the following ways, all of which makes it a concentration spell.
Auto Sentry - Fires a beam at any creature of your choosing that ends it's turn within 40ft of it. Dealing 1d6 damage of the type you chose when casting the spell. The construct no longer has its own turn.
Mobile Sentry - Gains a movement speed of 15ft which it can choose to move on it's turn. It is now vulnerable to force damage.
Arcane Sentry - The beam gains an effect based on the ability used to cast the spell. Intelligence, pushes a creature hit by the beam 10ft in a direct line away from it. Wisdom, pulls a creature 10ft in a direct line towards it. Charisma, reduces a creatures speed by 10ft. The beam only deals 1d8 of the damage type you chose when casting the spell.
When cast at higher levels, the construct fires one additional beam for each level higher than third. However if you chose to modify the spell to the Auto Sentry option, instead it's range increases by 10 ft for each level above third and deals 2d6, 3d6 and 4d6 when cast at levels 4, 6 and 8 respectively.
* - (A gemstone worth 50 gold which is consumed by the spell. )