I would like your advice about what I should bid in the following situation. Partner opens 1C, I answer 1H, he bids 2H. My hand is:
S : none
H : K Q J x x x
D : A K Q x x x
C : J
What should I bid now?
There is a convention called "exclusion blackwood" (some call it "voidwood"). This way you can ask for aces, excluding one suit.
This call can be made by bidding your void suit, in a level above your fit suit, provided that a fit has been found.
In your example, all bids (4s,5c,5d) are exclusion blackwood.
Partner can respond in steps, just like steps on 4NT(depends whether you play standard, blackwood, 1430RKCB, 3014RKCB etc). No matter what kind of blackwood you play, you will get your desired info (this is the two round aces).
You can look here for a variation of exclusion blackwood.
My concerns are:
Therefore I wish to take advantage of the opponents silence to describe my hand, so partner can make an intelligent bid again or double decision on the next round. Therefore I bid a simple 3D.
It is intended to give partner the greatest possible room to complete the description of his hand, which so far sounds like the only balanced hand at the table with 3-4-2-4.
It sounds like RHO is long in clubs and diamonds, and LHO is long in hearts and clubs, and no-one is holding better than honour-fourth in spades. At least two, if not three or four, spades are missing from this deck unless the opponents are red against white.
What is the vulnerability, and who is the dealer?
Update From OP above, both vulnerable and partner dealt.
Continuation. Partner will take 3D for a game try and bid one of:
Note how much easier the auction is if the original response is 1D receiving a raise; hearts can be cue-bid without ambiguity to locate the other missing card. Ease in correctly deciding between 5 or 6 r 7 is worth far more than the difference between a major or minor denomination.
The things your partner need most to know about are your spade void and strong diamonds. So I would bid three diamonds [changed from spades], to show my stopper in that suit.
If partner has "nothing" to say, he'll bid hearts, but more likely, he'll bid four clubs to show his ace in that suit. Then you can bid four spades [changed from diamonds] to show your void [changed from ace].
You have the material for a small, or even grand slam. All you need are the ace of clubs and the ace of hearts for the latter. Since partner opened, he will have one or both of those unless he has a lot of wasted honors in spades. (Hopefully he has the two good aces, eight high card points, plus a few, but not "many" spade honors for his opener).
Edit: Based on the comments of others, I believe I got the original order of suits wrong. That is, I now believe that you should bid three diamonds first to show the stopper, then spades to show the void. That is, lower-ranking suit, then higher.
From my comment to Thanos' answer:
Logically speaking, 3S splinter followed by 4NT over the expected 4H by partner should be keycard ask, without counting the Spade Ace (otherwise why not bid 4NT directly?). If partner bids 4D (likely singleton), you can still continue with a 4S bid and partner will know to take you further with the heart or club Ace.
So I would suggest bidding 3S, followed by 4NT over 4H.