D&D, more info = more tactics/strategy in battle; less info = more mystery. Balancing these two opposing things.

The following will be some thoughts, perhaps advice, to DMs and GMs running a table that likes tactical battles... but also likes the mystery/mystique/surprise/fear elements of fighting monsters in a fantasy settings. I feel like these can be competing needs that need a true balancing act to get right sometimes. 

Great mini painting by https://www.flickr.com/photos/citadelminis/2724201877/

In some of the recent posts, I talk about the Siabrae (custom undead druid for D&D) we had as a major miniboss right before Strahd... and then a large group of monsters that the party faced. There is no Siabrae class in D&D, so this miniboss was very unique and unknown to the party. How much of that uniqueness do you reveal to the party, so the party can make smart decisions? But how much is too much, where it is 100% a mathematical consideration... we still want to see the coolness factor of monsters and major bad guys in the game, right? 

Let's get to specific examples on how I tried, and I think successfully, balanced the information given to the party (the kid). 

• The Siabrae had one majorly hard-hitting spell, borrowed from Pathfinder 2e's Siabrae, called Eclipse Burst by them. It does I think 10d8 plus 10d4 damage to a very large area, 30 ft. radius. It hit 3 of the 4 PCs in the party. One or two of them went unconscious because of it. 

• Immediately I felt the need to tell the kid, "hey, I know this looks like curtains, but this guy can only cast it one time. He really pulled from deep to muster enough energy for this spell, he can't do it again [today]."
• Which lead to the kid having a sigh of relief. Now he knew it wasn't a "run away, far away, as fast as possible" situation, possibly leaving one or two PCs to die. He was still a little nervous, but he could make a plan and move forward. It worked out perfectly in the end. 

• On the flipside, early on the Siabrae's small army included a shambling mound which appeared from the ground. When it appeared, a faint green energy connection was seen between it and the Siabrae. This let the party know something was going on there, something was different, but no more information was revealed until the shambler died and the Siabrae (with a flash of green energy) fell to one knee, obviously hurt. Then I let the party know he took a pretty big hit. 
• This worked for us in this fight. There was no way the party could have known that would happen, but when it was revealed, it "made sense". The sense of wonder and mystery was felt and the situation was resolved.
• Another easter egg was the Siabrae kept using lightning spells to heal the Shambler, almost strangely incessantly. Again, once the reveal happened, it was like "oh THAT'S why he was healing that one guy so much instead of just attacking the party". 
• However, I could have seen a scenario where this piece of info was a fun option for the PCs to know and tactically decide to take on. I surveyed the battle and decided not to reveal to much this time. For example, if the kid/party was doing poorly, I could have revealed this information sooner so he would focus fire on the Shambler sooner. 
• Or, like many others (like SlyFlourish... BTW I just recently learned Sly Flourish is a rogue/thief ability in D&D 4e, interesting) have said and written about, sometimes it's cool to have a miniboss with say a ton of HP, and 3 totems/monsters or whatnot that sort of share HP. You give info to the players so that they make the tactical decision to go for one or the other. Less mystery, more tactics. 

So yeah, I see now it can't always be a "reveal everything"... but you definitely need to reveal some things, even if logically the PCs wouldn't know, so that a fun tactical battle can unfold. As a DM, give yourself some wiggle room... you don't want to reveal exact HP of every monster or the results are too obvious. But be fair and allow that tactical side of the fight to happen at least half the time... for us maybe 3/4 of the time. It's so much fun to know what options both you (the party) and the enemy have, so you (the party) can outsmart/outmaneuver the opponent, that we just go that route more often than not.