Mannahnin
Scion of Murgen (He/Him)
Thanks for this roundup.A few different spells use the word "familiar" (and there's probably more)....
Both locate object and locate creature use verbiage like:
Similarly, the spell clairvoyance refers to familiarity with respect to a location:
On the other hand, the scrying spell has a table defining the term:
There might be a little more nuance in the complete spell descriptions? But generally, the first few make it sound like a meeting in passing is sufficient; whereas the last one is specifically about more intimate acquaintance.
So... make of that what you will.
I rule Sending to function more like Locate Object, Locate Creature, and Clairvoyance. Passing familiarity is enough; you must have seen (I would allow heard/conversed with, for a blind spellcaster) and be able to identify the person.
I don't see any good basis for ruling that a Sending would disrupt a long rest. The recipient can respond and resume rest without any issues. As DM I might say that they wake briefly, or that they experience the Sending as a lucid dream, in which they are able to process the message and respond as if awake.Another question - what happens if the recipient is asleep? Do they 'receive' the message when they wake up and can reply then, or does the message wake them up? The spell description says 'can answer in a like manner immediately', which very tenuously implies the latter. But that'd be a fairly nasty way of fighting a known spellcaster in that case - organise your party to send Sendings to them every 7 hours for a few days to make sure they can't get a long rest, then come at them when their spells are depleted and when they've got a few levels of exhaustion stacked on.
A long rest is ruined by 1 hour of walking, any fighting, any spellcasting, or other similar strenuous activity.
Tangent, but I consider this a house rule. IMO the intent is clearly an hour total of strenuous activity; that the designers want PCs to be able to have a fight or two at night without it ruining the whole night's rest. Losing out on the Long Rest requires an extended interruption of an hour or more, and this is a deliberate change from older editions.