@
werecorpse
I have
bled the timelines of the Sundering (MiBG and LotCS) into ToD.
This accomplishes a few things
1. Elminster primarily concerned with assisting Mystra's return and the Weave's safety of the weave which explains his absence from ToD.
2. A division in the Lord's Alliance with Sir Isteval and Ulder Ravenguard on the one side versus the rest over what is of primary importance - for Isteval its the Chosen, for Ulder it is trying to save Baldur's Gate, while the rest are concerned with the ToD storyline.
3. The Emerald enclave is concerned over the growing cold and disturbances in the Icewind Dale.
Bleeding LMoP into ToD
The Spider is essentially a Cultist. The Wave Echo Cave was perhaps one of the hidden locations of the Dragonmasks.
Bleeding SKT in ToD
At the first Council Meeting it is suggested an alliance with the Giants (this also depends on the party's interaction with Blagothkus and if you intend running
Frozen Castle) would be helpful. This gets to the Cult's ears which leads to SKT.
SKT in the background creates turmoil amongst the giants and keeps the organisations such as the Harpers, Lord's Alliance and the Emerald Enclave very busy.
PCs are then forced to prioritize missions. The Council Meetings now become even more interesting as parties furiously debate and disagree on what is critically important.
Oota bleeds into ToD
Meanwhile, the Zhentarim have their own issues as they have been losing agents within the Underdark as Oota now bleeds into the campaign. They make a secret and rather generous plea to the PCs requesting their assistance.
Githyanki involvement
During the ToD you need to introduce a scouting party of Githyanki. They too have now become involved, given the historic Githyanki alliance with Dragons and Tiamat. With the effects of the Spellplague reversed and the returning of things to normality, the Githyanki have once more returned to Toril but this time at the bequest of Tiamat to assist her arrival on the Prime Material Plane.
Essentially the heroes need to defeat a few elite Githyanki in their quest to stop RoT (see below why).
Church of Tiamat
Severin, along with a few other top officials within the Cult, is secretly in the service of the Church of Tiamat, which explains how the Cult of Dragons core belief has become subverted. This could explain much of the in-fighting between the Cultists (Wearers of the Purple and some Wyrmspeakers).
ToD leads the PCs to
Palace of the Red Pasha module where the Church of Tiamat is exposed as the true culprit behind Severin.
Advice
The trick is to draw up a timeline and as the PCs make choices about quests to pursue throughout this multi-campaign/moduled saga things just begin unfolding in the background which keep all parties and organisations busy and essentially second-guessing themselves every step of the way as more issues rise up.
Campaign End
After the ToD storyline, run
The Lich Queen's Beloved
The module specifically states that Zetch'r'r solicits aid from a group of non-githyanki heroes who have effectively opposed his people in the past = the Githyanki elite in the ToD storyline.