Cheap blindsight

[J-H]

I ran the first part of a one-shot last night. Character level 11, introducing new players to D&D before going back to Against the Idol of the Sun.

Before the session:
Player: "I want a pet bat that can do recon for me."
Me (DM): "Ok, but if you want a magic bat that can talk to you or that you can give instructions to or that is smart enough to count, you need to use Find Familiar or pick up one of those magic figurines or something."
Player: "Ok, I'll just go with a regular bat then."

He brought it up again near the start of the session in person.

Partway through, some Quasits turned invisible and made a dash for the objective (it was a defense scenario, and they rode in on some Vrocks).

Player: "I open my coat, and cast Beast Sense."

Bats have 60' blindsight. Beast sense lets him share the bat's 60' blindsight (costs Concentration, so no Hunter's Mark), and the bat isn't doing anything except just sitting there on his shoulder or hanging off a pocket.

I thought it was very creative and liked it.

[Tanarii]

How did they persuade the bat not to freak out in the midst of combat and fly off? Precasting Speak with Animal?

Also don't forget it takes an Action to share senses through Beast Sense, and the caster can't use their own while using the beast's senses. Even if you let them still act while unable to see/hear, IMO at the minimum they can't see the other side of their body, and also should probably still take disadvantage to attacking due to the not seeing from their regular location / disorientation.

But still useful and innovative if they're just trying to find the invisible fiends.

[stoutstien]

How did they persuade the bat not to freak out in the midst of combat and fly off? Precasting Speak with Animal?

Also don't forget it takes an Action to share senses through Beast Sense, and the caster can't use their own while using the beast's senses. Even if you let them still act while unable to see/hear, IMO at the minimum they can't see the other side of their body, and also should probably still take disadvantage to attacking due to the not seeing from their regular location / disorientation.

True but beast bond could potentially pull it off.

[J-H]

How did they persuade the bat not to freak out in the midst of combat and fly off? Precasting Speak with Animal?

Also don't forget it takes an Action to share senses through Beast Sense, and the caster can't use their own while using the beast's senses. Even if you let them still act while unable to see/hear, IMO at the minimum they can't see the other side of their body, and also should probably still take disadvantage to attacking due to the not seeing from their regular location / disorientation.

But still useful and innovative if they're just trying to find the invisible fiends.

An 11th-level Ranger's pet is presumably pretty chill and trusting, and it was never directly threatened as the Vrocks were banished or distracted (the monk got eaten; mistakes were made).

I think we overlooked the "not getting your own senses" part, somewhat. After it was cast, the only thing he did was run and then shoot a couple of arrows. I should have applied disadvantage on the attack rolls, but they're Quasits, AC 12ish and 7hp, so it probably wouldn't have made much difference.

[Tanarii]

An 11th-level Ranger's pet is presumably pretty chill and trusting, and it was never directly threatened as the Vrocks were banished or distracted (the monk got eaten; mistakes were made).
If there needed to be justification, I guess it's not unreasonable they could have previously cast Speak with Animals to have some nice downtime chats with the beast. Possibly combined with using Wisdom (Animal Handling) to do some training. Personally I like that kind of thing to be clearly stated in advance, but I get that lots of tables will either back-justify explicitly. Or as you're doing, implicitly.

[Bobthewizard]

I'm not sure it's cheap. See invisibility is a second level spell that takes one action to cast and lasts for an hour without concentration.

Beast sense takes an action to cast, then an action to switch senses, and it uses your concentration. In the two rounds of getting the spell up, they probably could have just attacked the quasits with disadvantage and finished the fight.

But druids and rangers don't get see invisibility, so it could be a decent workaround if you wanted the spell otherwise.

[J-H]

If there needed to be justification, I guess it's not unreasonable they could have previously cast Speak with Animals to have some nice downtime chats with the beast. Possibly combined with using Wisdom (Animal Handling) to do some training. Personally I like that kind of thing to be clearly stated in advance, but I get that lots of tables will either back-justify explicitly. Or as you're doing, implicitly.

This is a one-shot, so backstory and "what happened earlier" are assumed and not written up. I try to be generous with "yes and" instead of "no."