Spoilers below:
Algenus was bewitched by a witch living in a cottage by a love potion, in order to fall in love and impregnate her
That was successful, the witch needed the child to kill it for its blood and use it for eternal youth potion (Mother Gothel, anyone?)
The twins were born and killed, but Linden, Algenus' wife, used her life-bound sword to revive them.
The book revolves also around Midsummer's hunt, by elves. Kind of "Lords and Ladies" by Pratchett vibe here, only the elves are basically good (with the exception of some hotheads) Oko transforms Algenus into a stag , then assumes the form of the elf queen to shoot him with an arrow, to incite war between elves of the Wilds and the Realm. In the end, Algenus is revived by his own life-bound sword that remained in the witch's cottage.
Along the course, Will saves Garruk from drowning and Garruk is cured from his curse by even stronger magic of Eldraine (some cauldron/well, I just skimmed through). He probably breaks from Oko's control before that, I have to read it thoroughly. The twins need Garruk to track the elven hunt and save their father (who is the target of the hunt after being turned into a stag).
Oko is revealed and almost subdued by Rowan, Will, Garruk & co. but he slips away. The Kenrith twins are revealed their true history and from the psychical trauma, their sparks ignite (together). Garruk, cured and well, promises the parents that he will track them and basically make sure they are safe. They helped him, he'll help them.
More details to follow.