If you have blood stones in a character's inventory, they need to be used to open a portal.
Where is Zixzax? Beside a portal, or the statue, or sitting near the weaver of time? If Zixzax is siting beside the weaver of time, you need to use another blood stone to open the next portal (or if you didn't already have all 16 star/blood stones, find another star stone). All of the portals need to be opened in the homestead before the door in the Source temple will open (except for the Forbidden Zone).
If you don't know, run around the homestead to see if he is standing beside a portal waiting for your characters (keep an eye on the mini-map for green dots). In particular, if you have not already opened the Hall of Darkness, check up the stairs to the right from the entrance of the homestead; he can be hard to see (with the default camera angle) if he is beside the portal at the end.
If he is beside the statue, you need to talk to the statue or enter the last opened elemental portal. The final elemental portal is the one opened just before the chapel portal, so likely the issue if you have most of the portals open.
Does the last opened portal have a marker on the map? If not, it wasn't completely opened. You need to enter, proceed far enough to get a cutscene conversation with Astarte or the demon, then return to the main section of the homestead through the portal.
You may be past this point, but if the homestead is still under attack, then it is by design that no portals are opening. You need to stop the fireballs raining down (doing something in the northern part of Luculla).