conjure sand sentinel

You call forth 3 sand sentinels to guard a location. Choose 3 areas at most with at least one gap no larger than 15 feet from each other. Each sentinel has 50 hp and 14 ac and is passive until provoked. your sand sentinels are there to guard from anyone trying to pass between them and will attack only if given warnings are ignored. 

When ignored the sentinels will spring into action, losing their humanoid form and turning onto an ooze-like ball of sand that attempts to grapple the intruders and triggering an alarm that will only be  heard by the caster. The grapple of the sentinels has a DC of 17 and can be avoided by either dex or str rolls higher than the number. 

This spell has no extra effects when using a high level spell slot and instead scales with how the caster uses it to the dm's discression. Ex. practicing casting faster to use in combat, finding new ways to create sentinels to make less resources needed or experimenting with different materials to give your sentinels new effects. The dm may also put downsides to practicing different styles in this way.